The error message is: The camera is locked by another application
It occurs for both of my Canon cameras 7D and G12, so nothing to do with them. It occurs since my upgrade to Ubuntu 20.04
If I cancel the error message I do get to view the images to transfer (but not select them separately) and can download all but after closing and restarting Shotwell all bar the last few downloaded images have vanished!
I have seen the other post re this problem where unmounting it from the file browser was the solution - it's not here!
I would say the problem is with Ubuntu 20.04 and Shotwell but I'm at a loss to investigate it any further.