Thanks to haddorp in SUPPORT-8274:

After migrating from 5.5.7 to 5.6.1 we have problems with the activation module when trying to delete a configuration node we get an error.

Please reference to 'error.log' attachment for more detail.

Problem caused by accessing the file publishing-core-backed-messages_en.properties.
"publishingMonitor.dateformat" is not found. So, the key instead of the actual value is used for the date formatter - and this does not work.
When putting a copy of this properties file on the class path it work. But I have to remove "_en" from the filename.
publishing-core/i18n/publishing-core-backed-messages.properties

Actually we just provided an '_en' file without fallback default one which easily cause issue to customers who don't use English as default languages.