Any problem that only occurs in one user account has a standard solution:
Your problem is caused by something in your user account. This could be:
third-party software that is triggered to launch automatically, or something like a Finder extension.
A setting or preference that is wrongly configured.
Something else that exists inside your
~/Library folder. (the ~ sign means the current user's home)
Check System Preferences > Users & Groups > Login Items to see what is set to load at Login. It's possible that one of those might be incompatible with Big Sur and causing problems. Check for updates. Disable all of them, reboot and test the problem.
Check Finder Extensions in System Preferences > Extensions. Test the problem with any third-party extensions disabled. (You'll need to reboot or restart Finder, at least.)
Check the LaunchAgents folder in the
~/Library. This may contain scripts that trigger background apps. These may need updating. Again, remove the plist files to another location, reboot and test.
If none of those things work, then you'll need to 'divide and conquer'. Start removing entire folders from the
~/Library, reboot and test. If the problem is still there, replace the folder. If it goes away, restore half the contents of that folder, and test again. Gradually, you'll focus on the problem.
Likely folders are the
Preferences folder, which contains ... well, preferences; and the
Containers folder, which contains individual apps' working files.
Note that this is all in the user's
Library, not the root level
Library. The user's
Library may be hidden in the Finder. Hold the Option key and click on the Go menu in the Finder, and Library should be a menu item.
Things not to do:
Don't reinstall the OS. The OS is working fine, as shown in the new user account. That will do nothing, except waste your time.
Don't migrate to the new user account. Changing user account can cause problems with existing file permissions. Also, if it happens again, will you keep moving on to new account after new account?