My hosting provider updated cPanel. I changed the PHP version from 8.2 to 8.3, and I enabled the same PHP extensions I had.
Since I did that, I keep getting the following error when I visit /admin/config/development/performance and I try to submit it.
The form has become outdated. Press the back button, copy any unsaved work in the form, and then reload the page.
Furthermore, the values I changed are not saved, and all the checkboxes are shown unselected.
When I visit another page, and return to that form, those values are shown as this screenshot shows.
When I submit another form, for example, the one on /admin/config/development/logging, I do not get any error, but the values are still not saved, even if the form shows all the values as before I changed them.
Do I need to delete some values from the database, for example from the cache_form table?
The issue was merely caused by Varnish, enabled by adding an enable_varnish file.
After I removed that file, the forms started to work again.