The latest update of qBittorrent brought HTTPS Certificate validation (a good thing), however it is not working properly on Mac OSX or Big Sur at this time.
If you update to these versions, and you use the HTTPS tracker (which is the default and preferred tracker), your qBittorrent will lose access.

You can fix it for now by disabling the certificate validation, which can be found:
preferences
advanced
near the bottom (12th from the bottom) will be "Validate HTTPS tracker certificates"

uncheck that, then close and relaunch qBittorrent. Should all fire up normally.

We've updated an issue on github about it: https://github.com/qbittorrent/qBittorrent/issues/14610

We'll also update again when/if it gets fixed and you can either safely update or enable the validation again.