Hi @Adrian Kaczyński ,
Glad to hear this issue has been resolved!
Thanks for the sharing.
However, due to a recent update in forum policy, the question author now is not able to accept their own answers.
So I have written a brief summary of the solution this issue. Please feel free to accept it as the answer, which would benefit others who also has similar issues in forum.
Issue:
Some users who use Outlook classic (2019, 2021) on desktops have been encountering problems with syncing folders. Most notably it regards the "Sent" folder. During "Send/Receive" Outlook hangs up on subscribing to folders and this action takes 10 to 20 minutes.
Solution:
OP used the sync tool provided by Google called GWSMO. Using this on the user's computer worked like a charm and fixed the Outlook problem. Guide for using this tool is available at https://support.google.com/a/users/answer/171710