Hey there, PaulvanBox
Welcome to the Microsoft Community forums.
You could try unlinking and relinking your account on OneDrive. To do that, please:
- Go to the OneDrive Icon on the Taskbar Icon area,
- Right click onto OneDrive,
- Click onto settings,
- Click onto Account Tab,
- and unlink from this PC.
After you have done this, please go back and relink your account and see if your Excel Document saves.
I do, also, have some additional questions to see if we can find out why this is happening.
- Did you have an update prior to this?
- Is it only your Excel program that is signed out?
- Has any settings been changed in the Office programs?
Hope this helps,
Regards,