Using all latest versions : 2.15.5.
Every time visiting Global Config Files: log has exception:

WARNING: Caught exception evaluating: it.groupedConfigs in /configfiles/index. Reason: java.lang.reflect.InvocationTargetException
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
....
Caused by: java.lang.IllegalStateException: class org.jenkinsci.lib.configprovider.model.Config must override 'getDescriptor()' this method!
at org.jenkinsci.lib.configprovider.model.Config.getDescriptor(Config.java:108)
at org.jenkinsci.lib.configprovider.model.Config.getProvider(Config.java:115)
at org.jenkinsci.plugins.configfiles.GlobalConfigFiles.getGroupedConfigs(GlobalConfigFiles.java:72)
at org.jenkinsci.plugins.configfiles.ConfigFilesManagement.getGroupedConfigs(ConfigFilesManagement.java:112)
... 116 more

Do not know why but file : org.jenkinsci.plugins.configfiles.GlobalConfigFiles.xml
for newly created items has

    <org.jenkinsci.plugins.configfiles.custom.CustomConfig>
      <id>5aa81ca6-1ed0-4444-bded-03960d58fc46</id>
      <name>MyCustom</name>
      <comment></comment>
      <content>342342343</content>
      <providerId>org.jenkinsci.plugins.configfiles.custom.CustomConfig</providerId>
    </org.jenkinsci.plugins.configfiles.custom.CustomConfig>

for old

    <org.jenkinsci.lib.configprovider.model.Config>
      <id>id_config_1</id>
      <name>name_1</name>
      <comment>comment_1</comment>
      <content>content_1</content>
      <providerId>org.jenkinsci.plugins.configfiles.custom.CustomConfig</providerId>
    </org.jenkinsci.lib.configprovider.model.Config>