Jenkins log shows the following on boot;  the plugin is still working so this is more of an FYI

Attempt to (de-)serialize anonymous class org.jenkinsci.plugins.configfiles.GlobalConfigFiles$1 in file:/var/lib/jenkins/plugins/config-file-provider/WEB-INF/lib/config-file-provider.jar; see: https://jenkins.io/redirect/serialization-of-anonymous-classes/

The instructions include a command to locate the offending lines, here's the output:

javap -classpath /var/lib/jenkins/plugins/config-file-provider/WEB-INF/lib/config-file-provider.jar -l 'org.jenkinsci.plugins.configfiles.GlobalConfigFiles$1'
Compiled from "GlobalConfigFiles.java"
final class org.jenkinsci.plugins.configfiles.GlobalConfigFiles$1 implements java.util.Comparator<org.jenkinsci.lib.configprovider.model.Config> {
org.jenkinsci.plugins.configfiles.GlobalConfigFiles$1();
LineNumberTable:
line 30: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lorg/jenkinsci/plugins/configfiles/GlobalConfigFiles$1;

public int compare(org.jenkinsci.lib.configprovider.model.Config, org.jenkinsci.lib.configprovider.model.Config);
LineNumberTable:
line 33: 0
LocalVariableTable:
Start Length Slot Name Signature
0 12 0 this Lorg/jenkinsci/plugins/configfiles/GlobalConfigFiles$1;
0 12 1 o1 Lorg/jenkinsci/lib/configprovider/model/Config;
0 12 2 o2 Lorg/jenkinsci/lib/configprovider/model/Config;

public int compare(java.lang.Object, java.lang.Object);
LineNumberTable:
line 30: 0
LocalVariableTable:
Start Length Slot Name Signature
0 13 0 this Lorg/jenkinsci/plugins/configfiles/GlobalConfigFiles$1;
}