Appian platform Tomcat error: Missing authentication token

In the Tomcat-log of my on-prem Appian platform I see the following error approximately once a day and not on all machines of my Appian cluster:

00:20:00,050 [com.appiancorp.quartz.BackgroundJobScheduler_Worker-1] ERROR com.appiancorp.plugins.backups.quartz.PluginCleanupJob - Error cleaning up old backup plugin files java.lang.IllegalStateException: Missing authentication token.

Yes, I tried searching the Appian Community, but maybe somebody here has any idea how to figure out which token is supposidely missing (for which system)? The curious thing is that all Appian plugins seem to work without issues and why this error is only occuring on one (and not all machines) of the cluster.