Hello Olga Dolder-Prozorovskaya ,

This is likely to be a permissions issue and that you are not running Studio as admin.

Please restart Studio by right-clicking and selecting Run As Administrator.

I would also suggest (at least temporarily) deactivating the sync with your One Drive folder before running Studio.

I have the same problem too. I upgraded my Trados studio Freelance 2019 to the latest 2021 and when I try to save my project, I receive the same error. I´m stuck with this problem for 2 weeks already!!!! Moreover, in my notifications I have a message saying I don´t have internet access although my internet is working perfectly....

Rename this folder (just add _old to the end or something, it's not important what you rename it to) and restart Studio:

c:\Users\[USERNAME]\AppData\Roaming\SDL\SDL Trados Studio\14.0.0.0

rename de folder [drive]:\windows\users\[username]\OneDrive\Documents\Studio 2021 (if you use or used Microsoft OneDrive in the past] or the folder [drive]:\windows\users\[username]\Documents\Studio 2021 for anything else. 

Rename 

C:\Users\olgad\OneDrive\Documents/Studio2021/AutoSave/4c1096a1 to 

C:\Users\olgad\OneDrive\Documents/Studio2021/AutoSave/old_4c1096a1

Rename 

C:\Users\olgad\OneDrive\Documents/Studio2021/AutoSave/4c1096a1 to 

C:\Users\olgad\OneDrive\Documents/Studio2021/AutoSave/old_4c1096a1