Have you opened up your projects.xml file (in Documents > SDL Studio 2019 > Projects by default). One possibility could be that your projects.xml file is corrupted (I had problems with mine when I ran out of space in my Documents folder). 

Thanks for your quick reply Michael. I can open the projects.xml fine. I noticed that the actual projects are no longer being recognised as Studio projects (blank logo) which wasn't the case this morning.

I'm also wondering why the additional information in the error message seems to be referring to a non-existent OneDrive file.

Thanks anyway for your suggestion.

Hello Christine Hokamp ,

Have you tried resetting Studio?

https://gateway.rws.com/csm?id=kb_article_view&sysparm_article=KB0033669

Also, you mention blank logos/icons.

Where are you seeing these? In your projects folder?

Are you able to post a screenshot of what you are seeing please.

Yes, the sdlproj icon is normally recognizable as an sdl file but not below. When I tried directly opening projects this morning (as opposed to Studio first) the usual sdl project icon was visible. Maybe I made it worse by trying some other workarounds.

I'm currently copying projects, TMs etc. and will then try resetting / reinstalling Studio.

Thanks for the link above - I'll try that.

The problem is solved, although I have no idea how.

When going to uninstall Studio 2019 I noticed an old Studio 2017 suite which I am sure was already removed and I didn't see this morning when checking things like old installations. I uninstalled that and got a message about being sure to return license unless I wanted to reinstall on this computer. I then tried Studio 2019 again which started up - although only with very old projects. I was able to add the most recent projects by hand.

Thank you to all who replied and also to those in the background who gave the situation their thoughts.