Solved!
I looked into the theme a little further and found a strange line in function.php that was causing this problem.
I changed it and now it works properly again, even with Twenty Twenty.
Thanks for your help!
I created a staging website and checked everything, and it has nothing to do with a plugin.
I installed a different theme, and that solved the problem. The theme where the problem occurs is Twenty Twenty from WordPress. Does this also happen with your clean installation when you use this theme?
Based on this information, is there a solution? Installing a completely new theme is not preferable because it messes up the entire layout.
Thank you for your quick response.
There is no error log, so I cannot share it.
You can find the status page in the following link. I hope this clarifies things.
https://pastebin.com/1EWXBpQV
