The message often appears when a server-side error occurred during saving. I have the following recommendations:
Deactivate all plugins for the time being. Can you then save? If yes, it was due to one of the deactivated plugins. You can find out which one it was by activating it bit by bit and testing it.
If deactivating all plugins doesn’t help either, you should check the error log of your hosting to see what it says. The support of your hoster can certainly help you.
