Update: as soon as i hit the post button I received the sd card message again.
Some Ideas. One should NOT buy cards on Capacity alone. As the phones get faster the speed of the card becomes more important. Please know taking a SLOWER card from an old phone that worked perfectly MAY cause problems if it is put into a new phone. Reseating the card also makes sense. Cost of cards is highly dependent on the speed. If you replace the card make sure it is faster than old one.
Following this thread as I'm encountering this same issue with the S20+. I've never had the same problem with my previous Note9. I encounter this every few weeks and restarting the phone fixes the issue.
However, just this morning, my phone encountered the same error and won't read my card anymore. I tried restarting at first, didn't fix it this time. Tried restarting and removing the card, still not fixed. Finally, I inserted the card to my PC, and voila, it worked, so I'm creating a backup now in my PC.
My card is also a U3. And seems the software updates are the only recent changes I've done in my phone since the card was inserted there since I bought the phone last year, and have not removed it ever since.