It seems that the following commit (from EZP-21547 ) introduces an incorrect error handling:
https://github.com/ezsystems/ezpublish-legacy/commit/57171cd9170eb7384579a2b29549e2a37fbb002c

This will cause eZ Publish to fail while handling the errors, and when an error needs to be displayed it will fail, presenting a completely different error, and not the one it should.