Veit Guenther 

This is quite a unique situation I think, one that doesn't come around often!  It seems possible that OneDrive or a similar network share is at play here, with files set to 'cloud-only' mode. This type of setup might be why Studio is unable to locate and work with these files as it normally would since they may not really be available. I struggle to envisage any other explanation for this extraordinary behaviour.

It might be worth a moment to check your drive settings to ensure they're not contributing to this anomaly.

Veit Guenther 

This is quite a unique situation I think, one that doesn't come around often!  It seems possible that OneDrive or a similar network share is at play here, with files set to 'cloud-only' mode. This type of setup might be why Studio is unable to locate and work with these files as it normally would since they may not really be available. I struggle to envisage any other explanation for this extraordinary behaviour.

It might be worth a moment to check your drive settings to ensure they're not contributing to this anomaly.

Eventually I found a solution, which consisted in reorganizing the termbase in Multiterm. I worked from then on for a few hours and then stopped working. After reorganizing, the termbase had grown from 160MB to 850MB and it was probably too unwieldy to be handled by Studio. Fortunately I had the original termbase and I was able to start the process from scratch again.

A new, similar issue appeared today: A TM (TM results) works just for one segment. After confiming the segment, the TM gets detached and is not available for the translation of subsequent segments.

Workaround: Re-attach TM after every segment (which is not a viable solution)

I have stored all projects files, including the TM, on a local drive (not a cloud drive), tried re-indexation as well.