I just pushed this commit to core:
It introduces a new rake task which should help us track down this issue. We’re not seeing any errors at all on our hosting, so that suggests it’s related to a third-party plugin.
Please can someone with the issue update to the latest tests-passed, then run:
./launcher enter app
cd /var/www/discourse
bin/rake "revisions:debug_deserialization"
That should print out information about the problematic classes, and one example revision for each class. Please share the summary/example section of the output in this topic (or via PM, if you prefer not to share the example revision publicly).
Once we know exactly what’s causing the issue, that will help us build a fix. (likely to be either a database migration in the third-party plugin, or a core rake task which can cleanup any unexpected data)