Quicktime for Big Sur

i think you should try and create a test account to see if the issue is system wide, or is isolated to your main account. please see this link if you need instructions to make the account: Set up users, guests, and groups on Mac - Apple Support.

if the issue does not occur in the test user account, that means the issue is likely due to some incompatible software in your main account. if that's the case, i'm thinking you should download and run the free version of EtreCheck so we can see if you have some software installed that is causing your issue. make sure you give "full disk access" to etrecheck. read how to use it by reading Using EtreCheck. if you need help interpreting the report, you can see how to post the report here by reading How to use the Add Text Feature When Posting Large Amounts of Text, i.e. an Etrecheck Report. and it automatically obscures sensitive things (like serial numbers) so you don't have to worry about sharing the report here.

OT has been performing as usual for me since i upgraded to big sur last fall. what is going wrong (not working as expected) in your instance? are you getting any error messages? and what version of QT is installed on your mac?