Browser: Chrome Version 148.0.7778.168 (Official Build) (arm64)
OS: macOS
URL: desolhair.gr
We are not able to make changes because the save never finilizes and we get a 504 Error.
All plugins are deactivated. but still
Found some other posts but there was no info to help us with this issue 
504 errors are mostly server-side issues, so you need to check with your hosting provider.
It was an issue with Bricks and Wordpress 7 autoupdate.
We installed Bricks and solved the issue for some reason!
Thnak you for your answer!!
Cool, glad you sorted it out.
Just as an aside, if it auto updated and crapped out during the update, I wonder if you had enough resources for it on the server? Just a thought for the future.