Apple made major changes to Time Machine in Big Sur, so backing up to a Time Capsule is no longer a reliable way to back up the data on your Mac for many users......including me.
There are reasons for this, starting with the fact that Big Sur reformatted your Mac's hard drive when you installed Big Sur, but the Time Capsule cannot be formatted in the same way that your Mac's hard drive is now set up. So, Time Machine has to try to back up from your Mac using one format to the Time Capsule by changing the format of the Time Capsule backup. Gosh, I wonder what could could possibly go wrong?
I would suggest that you look at other methods to back up your Mac.
The simplest, least expensive, fastest and most reliable way to back up using Time Machine is connect a USB hard drive directly to your Mac. I've had zero problems since I switched to this type of backup shortly after Big Sur was introduced.
If you want to continue to use the Time Capsule, try backing up using Carbon Copy Cloner. I've had zero problems with this type of back up as well.