During Jenkins startup I get:

WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration#metaClass: type is abstract but not Describable.

=> Extracted/moved the problem from (very generic issue) https://github.com/jenkinsci/configuration-as-code-plugin/issues/536

It seems that after updating JCasC plugin from 1.31 to 1.32 a similar warning appears during Jenkins startup that was not there before:

2019-10-10 14:56:50.519+0000 [id=119]   WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration#metaClass: type is abstract but not Describable.

(Job DSL plugin is installed in version 1.76 since ~ end of August)

Unfortunately it (the plugin update as root cause) does not make fully sense though, as the warning did not appear right after first startup after plugin update. And I updated a few other plugins at the same time (e.g. "script-security" from 1.64 to 1.66) and about 10h before Jenkins core from 2.198 to 2.199... And changed the job DSL usage in JCasC configuration also about at the same time as the Jenkins core upgrade...

Actually, according to the Jenkins logs:

...
2019-10-10 04:48:14.488+0000 [id=29]    INFO    hudson.PluginManager#loadDetachedPlugins: Upgrading Jenkins. The last running version was 2.198. This Jenkins is version 2.199.
2019-10-10 04:48:14.532+0000 [id=29]    INFO    hudson.PluginManager#loadDetachedPlugins: Upgraded Jenkins from version 2.198 to version 2.199. Loaded detached plugins (and dependencies): []
...
2019-10-10 04:48:47.404+0000 [id=23]    INFO    hudson.WebAppMain$3#run: Jenkins is fully up and running
...
2019-10-10 14:50:14.283+0000 [id=82]    INFO    h.m.UpdateCenter$UpdateCenterConfiguration#download: Downloading configuration-as-code
---> Plugins installed:
        script-security:1.66
        docker-workflow:1.21
        pam-auth:1.6
        email-ext:2.68
        htmlpublisher:1.21
        mailer:1.29
        ssh-credentials:1.18
        configuration-as-code:1.32
---> Initializing Jenkins restart after plugin installation!
2019-10-10 14:50:14.856+0000 [id=30]    INFO    j.util.groovy.GroovyHookScript#execute: Executing /var/lib/jenkins/init.groovy.d/03-basics.groovy
2019-10-10 14:50:14.869+0000 [id=86]    INFO    jenkins.model.Jenkins$23#run: Restart in 10 seconds
--> Jenkins restart is pending: skipping this hook script
2019-10-10 14:50:14.897+0000 [id=30]    INFO    jenkins.InitReactorRunner$1#onAttained: Completed initialization
2019-10-10 14:50:14.928+0000 [id=23]    INFO    hudson.WebAppMain$3#run: Jenkins is fully up and running
2019-10-10 14:50:24.869+0000 [id=86]    INFO    jenkins.model.Jenkins$23#run: Restarting VM as requested by SYSTEM
2019-10-10 14:50:24.870+0000 [id=86]    INFO    jenkins.model.Jenkins#cleanUp: Stopping Jenkins
2019-10-10 14:50:24.880+0000 [id=86]    INFO    jenkins.model.Jenkins$18#onAttained: Started termination
2019-10-10 14:50:24.900+0000 [id=86]    INFO    jenkins.model.Jenkins$18#onAttained: Completed termination
2019-10-10 14:50:24.900+0000 [id=86]    INFO    jenkins.model.Jenkins#_cleanUpDisconnectComputers: Starting node disconnection
2019-10-10 14:50:25.046+0000 [id=86]    INFO    jenkins.model.Jenkins#_cleanUpShutdownPluginManager: Stopping plugin manager
2019-10-10 14:50:25.059+0000 [id=86]    INFO    jenkins.model.Jenkins#_cleanUpPersistQueue: Persisting build queue
2019-10-10 14:50:25.101+0000 [id=86]    INFO    jenkins.model.Jenkins#_cleanUpAwaitDisconnects: Waiting for node disconnection completion
2019-10-10 14:50:25.101+0000 [id=86]    INFO    jenkins.model.Jenkins#cleanUp: Jenkins stopped
...
2019-10-10 14:50:45.437+0000 [id=23]    INFO    hudson.WebAppMain$3#run: Jenkins is fully up and running
...
2019-10-10 14:51:46.712+0000 [id=19]    INFO    hudson.PluginManager#install: Starting installation of a batch of 5 plugins plus their dependencies
2019-10-10 14:51:47.569+0000 [id=112]   INFO    h.model.UpdateCenter$DownloadJob#run: Starting the installation of xunit on behalf of r.fuereder
2019-10-10 14:51:48.189+0000 [id=112]   INFO    h.m.UpdateCenter$UpdateCenterConfiguration#download: Downloading xunit
2019-10-10 14:51:48.330+0000 [id=112]   INFO    h.model.UpdateCenter$DownloadJob#run: Starting the installation of windows-slaves on behalf of r.fuereder
2019-10-10 14:51:48.914+0000 [id=112]   INFO    h.m.UpdateCenter$UpdateCenterConfiguration#download: Downloading windows-slaves
2019-10-10 14:51:49.108+0000 [id=112]   INFO    h.model.UpdateCenter$DownloadJob#run: Starting the installation of github on behalf of r.fuereder
2019-10-10 14:51:49.682+0000 [id=112]   INFO    h.m.UpdateCenter$UpdateCenterConfiguration#download: Downloading github
2019-10-10 14:51:50.323+0000 [id=112]   INFO    h.model.UpdateCenter$DownloadJob#run: Starting the installation of jackson2-api on behalf of r.fuereder
2019-10-10 14:51:50.892+0000 [id=112]   INFO    h.m.UpdateCenter$UpdateCenterConfiguration#download: Downloading jackson2-api
2019-10-10 14:51:51.439+0000 [id=112]   INFO    h.model.UpdateCenter$DownloadJob#run: Starting the installation of apache-httpcomponents-client-4-api on behalf of r.fuereder
2019-10-10 14:51:52.019+0000 [id=112]   INFO    h.m.UpdateCenter$UpdateCenterConfiguration#download: Downloading apache-httpcomponents-client-4-api
...
2019-10-10 14:52:25.118+0000 [id=23]    INFO    hudson.WebAppMain$3#run: Jenkins is fully up and running
...
2019-10-10 14:56:50.449+0000 [id=119]   WARNING hudson.util.Secret#toString: Use of toString() on hudson.util.Secret from hudson.util.Secret$1.convert(Secret.java:306). Prefer getPlainText() or getEncryptedValue() depending your needs. see ht
tps://jenkins.io/redirect/hudson.util.Secret/
2019-10-10 14:56:50.454+0000 [id=119]   WARNING hudson.util.Secret#toString: Use of toString() on hudson.util.Secret from hudson.util.Secret$1.convert(Secret.java:306). Prefer getPlainText() or getEncryptedValue() depending your needs. see ht
tps://jenkins.io/redirect/hudson.util.Secret/
2019-10-10 14:56:50.454+0000 [id=119]   WARNING hudson.util.Secret#toString: Use of toString() on hudson.util.Secret from org.apache.commons.beanutils.converters.AbstractConverter.convertToString(AbstractConverter.java:195). Prefer getPlainTe
xt() or getEncryptedValue() depending your needs. see https://jenkins.io/redirect/hudson.util.Secret/
2019-10-10 14:56:50.457+0000 [id=119]   WARNING hudson.util.Secret#toString: Use of toString() on hudson.util.Secret from hudson.util.Secret$1.convert(Secret.java:306). Prefer getPlainText() or getEncryptedValue() depending your needs. see ht
tps://jenkins.io/redirect/hudson.util.Secret/
2019-10-10 14:56:50.457+0000 [id=119]   WARNING hudson.util.Secret#toString: Use of toString() on hudson.util.Secret from org.apache.commons.beanutils.converters.AbstractConverter.convertToString(AbstractConverter.java:195). Prefer getPlainTe
xt() or getEncryptedValue() depending your needs. see https://jenkins.io/redirect/hudson.util.Secret/
2019-10-10 14:56:50.459+0000 [id=119]   WARNING hudson.util.Secret#toString: Use of toString() on hudson.util.Secret from hudson.util.Secret$1.convert(Secret.java:306). Prefer getPlainText() or getEncryptedValue() depending your needs. see ht
tps://jenkins.io/redirect/hudson.util.Secret/
2019-10-10 14:56:50.459+0000 [id=119]   WARNING hudson.util.Secret#toString: Use of toString() on hudson.util.Secret from org.apache.commons.beanutils.converters.AbstractConverter.convertToString(AbstractConverter.java:195). Prefer getPlainTe
xt() or getEncryptedValue() depending your needs. see https://jenkins.io/redirect/hudson.util.Secret/
2019-10-10 14:56:50.480+0000 [id=119]   WARNING hudson.util.Secret#toString: Use of toString() on hudson.util.Secret from hudson.util.Secret$1.convert(Secret.java:306). Prefer getPlainText() or getEncryptedValue() depending your needs. see ht
tps://jenkins.io/redirect/hudson.util.Secret/
2019-10-10 14:56:50.481+0000 [id=119]   WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class jenkins.security.plugins.ldap.FromGroupSearchLDAPGroupMembershipStrategy#authoritiesPopulator: type is abstract but not Describabl
e.
2019-10-10 14:56:50.519+0000 [id=119]   WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration#metaClass: type is abstract but not Describable.
2019-10-10 14:56:50.710+0000 [id=119]   WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class jenkins.scm.impl.subversion.SubversionSCMSource#owner: type is abstract but not Describable.
...

... it seems as if the first time was caused by going to "Manage Jenkins > Configuration as Code" and then "View configuration"!?

$ grep "Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration" /var/log/jenkins/jenkins.log
2019-10-10 14:56:50.519+0000 [id=119]   WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration#metaClass: type is abstract but not Describable.
2019-10-11 04:34:36.361+0000 [id=35]    WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration#metaClass: type is abstract but not Describable.
2019-10-11 04:34:40.194+0000 [id=35]    WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration#metaClass: type is abstract but not Describable.
2019-10-11 04:39:59.751+0000 [id=45]    WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration#metaClass: type is abstract but not Describable.
2019-10-11 04:40:02.857+0000 [id=45]    WARNING i.j.p.casc.BaseConfigurator#createAttribute: Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration#metaClass: type is abstract but not Describable.

$ grep "Can't handle class javaposse.jobdsl.plugin.GlobalJobDslSecurityConfiguration" /var/log/jenkins/jenkins.log.1
# Nothing

(Sorry for this rather weird comment due to trying to give the full picture from the top of my head)