As you moved over your old home of Ubuntu 18.04 to Ubuntu 20.04, and apparently by hand, it might be that your old user configuration data, i.e., the hidden files, do not play well with Ubuntu 20.04. You can test that by (temporarily) creating a new user account. Very likely, the new account will not have the issue.
If this way, you have confirmed that the user configuration data is the problem, then you can try resetting your main account as indicated here. Before proceeding, to be on the safe side and keep having a working account, you can give administrator privileges to the temporary user you have created. Instead of having to boot into a recovery prompt, you can use the account of your temporary new user to do this.
If, after the procedure, you succeed to log in into your current account, you can move the user data (Documents, Pictures, ...) back. Once everything is in place, the temporary account can be removed again and the
/home/youruser-backup folder can be deleted.