Hi Maik,
The bug you're hitting is this: Changing a user's email - bug - Discourse Meta
But there's a workaround here: On email address change second confirm fails due to email customization - #21 by pfaffman - support - Discourse Meta
The software is sending “confirm your new email address” emails with links like https://forum.xx.org/users/authorize-email/c7ed779eb28c049ec4a5105dd9e07013 rather than https://forum.xx.org/u/confirm-new-email/c15d4bc33211e586b70572195f7dac89.
So if you get that URL with authorize-email in the URL, change URL that to contain confirm-new-email and reload it.