Hi Moritz,
It is very unfortunate that the issue comes back again. This might be some other reason behind this, I would suggest you troubleshoot the following common issues to recognize the reason.
1.Plugin Conflict
2.Theme Update related issue
3.CSS/JS Customization
4.Disabling Cache
5.Disabling CDN
If none of the above helps and you are still getting the issue, please copy your live site to a staging server so we could troubleshoot freely without breaking your live site.
And give us access in the secure note including:
– WordPress Site URL & Login URL
– WordPress Admin username/password
To create a secure note, click the key icon underneath any of your posts.
Thanks