I have upgraded to Big Sur and there are no current problems, as such, just one little quirk. After the main upgrade a subsequent software update came through. So what you now have to do if you run Big Sur with the latest update (as of this date) on your Apple iOS is:
1. Ensure your dongle is plugged in before you start the computer. If you are finding you have to sign documents etc. midway through the day you will have to shut down, plug in and then restart.
2. Once you have done this, with the latest Big Sur update you will as usual be prompted to enter your PIN to sign documents. The quirk is you must now do this for every signing you must undertake. So, if, for example, there are "6 ticks" next to things you have to sign off then you will be asked 6 times to enter your PIN. This is a little annoying but scarcely life threatening.
Hope this assists.
