Unable to save changes on existing pages

Hello,
I have been experiencing recurring issues for quite some time with saving changes on existing pages. The changes are not applied after saving.
I have already tried the following troubleshooting steps:
– Tested in different browsers
– Saved the page as a draft
– Disabled all plugins
– Tried saving the content as a template and inserting it into a test page (the template is not saved either)
The WordPress installation as well as the theme are fully up to date.
Due to the frequency of this issue and its impact on productive work, this matter is becoming increasingly urgent. I therefore need not only a fix for the root cause, but also a reliable workaround, as the problem keeps occurring.

Thank you in advance for your support and assistance.

Best regards, Kornelia