@Kate: Thank you very much for your post above. I did a search to see if I could find the solution you mentioned. I did. It's here:
viewtopic.php?f=4&t=121992#p699837Thanks also to Edward for coming up with that solution. Not sure how he managed that, but happy he did.
I was able to reproduce the scenario on my system again, and verified that Edward's solution worked for me.
So, anyone that does a Big Sur update, and then finds that Resolve hangs instantly when they play the timeline with a BMD I/O device connected and selected, or see Media Express hang a few seconds after playing a clip, Edward's solution at the link above appears to be something to try. While the process is a bit painful, it's probably easier than doing a clean Big Sur install from scratch, which was our previous solution.
Just to note, this process is ONLY if Desktop Video sees your I/O device, and you have the playback issues mentioned above. It's not for a scenario where Desktop Video does not see your I/O device at all.