This started happening to me, as well, when I recently went to use AirDrop to send photos from my iPhone to my Mac Studio, something I do all the time. I also discovered, during my troubleshooting, that my iCloud drive wasn't syncing either when I tried to move them that way. I eliminating my iPhone as a suspect by successfully AirDropping some images to my MacBook Air. None of the solutions I found here helped.
What did work was signing out of my iCloud account on my Mac, opting to not keep any files on my Mac, and then signing back in. That seemed to solve both the AirDrop and the iCloud file synch problems.
One important note: after logging back in, using the Mac Studio icon for AirDrop still didn't work at first. I had to use the AirDrop icon to share the file the first time. (Then using the Mac icon worked.)