Wordpress: 5.2.3
Ultimate Blocks: 2.1.3
I have reinstalled wordpress, the plugin, deactivated other plugins – nothing helped. But I still found the solution for my case. Thing is that I turned on error notifications in php for logged in users. If there is any output before the header, redirections usually doesn’t work but with gutenberg enabled (since 5.0) wordpress doesn’t even save the post at all. Maybe it has something to do with the rest api that is canceled for the same reason? If I deactivate gutenberg I can save posts even if there are “errors” – notices, warning etc.
So the error/notice is still there and caused by the tab system only – BUT if I disable the notifications I can save the post and it still works. Maybe this information helps you and others.