keynote doesn't save changes

Same problem for me. Keynote won't save its files anymore. 'The document could not be automatically saved / could not be opened now. The current changes are possibly lost.'

I am quite concerned - presenting is an important part of my work.

I searched the internet and already tried several things:

1. Save the file on a different location. Or under a different name.
2. Restart in the safe mode. That helped - but only for 10 min.
3. Update the OS and the app. But they were already up to date: macOS Sonoma 14.4.1, Keynote 14.0
4. Re-install Keynote.
5. Deactivate unused fonts in the Font Catalog. That helped - but only for 30 min.

Saving the same files, however, still DOES work on my old laptop. That still runs under macOS Monterey 12.7.4, with Keynote 13.1.

Do you know how to fix this? Thanks!

Problem solved. The antivirus software I use was blocking the access of Keynote to the folder where it stores a.o. automatically saved files.

The antivirus now came with a popup window:

'Activity blocked. Keynote tried to gain access to the folder "Library", but has been blocked according to the policy of DeepGuard."

The window, however, offered the option to allow access. Which worked. Also, the history of the blocking was logged in the antivirus app.

The antivirus software is called 'KPN Veilig Virusscanner' and is coming from F-Secure. The current version is 19.5.