If you search for this issue, you’ll find many results on the net regarding different reasons.
But, if you didn’t get into a result solving your case, you may need to check your user profile. If it’s not located on the default location or moved like I did in this article you’ll end up with above issue.
You can check this https://docs.microsoft.com/en-us/microsoftteams/known-issues for more details.
However, a workaround can be done by logging to another account that has the profile done properly, then copy the Teams folder from %userprofile%\AppData\Local\Microsoft to another location and create a shortcut for the Teams.exe file.
Although this will solve the startup issue as well as most of the functions, it won’t solve the download issue. Because the download folder is part of the profile.
So, you’ll need to right-click Downloads folder, select properties, and finally redirect the file location to the exact current location instead of using Symlink.