MyCloudEX2Ultra freezes with MacBook using Catalina

I have a MyCloudEX2Ultra that has been working happily as Time Machine Backup for a couple of years until a week ago. Now it gets so far and then just freezes about a quarter of the way along the progress bar, and still showing “Preparing backup…”
I’m a non techy, and just need it to work… I’ve been on Catalina for a couple of months.
It shows in the Finder sidebar. I’ve 'selected the disk several times on Time Machine. Disk Utility is saying it’s ‘not mounted’ and greyed out whatever that means. I press “mount” and I get one of those crazy error messages ’ Could not mount “Time Machine Backups”. ( error -119930868.)