Yes, the conflict still exists. Nothing changed with deactivating all plugins.
I guess it has something to do with my multilanguage website (using the Polylang plugin). But on another multilanguage website of mine, the conflict does not exist and everything is fine. So I am just guessing and don’t have any clue what’s wrong.
Deleting the Ultimate Member plugin with all data and then reinstalling it also doesn’t change anything and the problem still occurs.