Encryption paused, can't resume, cancel or mount disk

Thanks, Joseph. The page you linked to gives instructions for repairing a disk using Disk Utility, which I've already tried. fsck_apfs reports that the volume "appears to be OK". Since no errors are found, the utility doesn't try to fix anything when invoked as "fsck_apfs -y -x". However, encryption doesn't resume, and the volume still can't be mounted.

Was there anything else on the linked page that I missed?

Thanks, HWTech. As I explained in my reply to Joseph, I've already tried Disk Utility First Aid, as well as running fsck_apfs from the command line. No errors are found, and there is nothing to repair. You are correct that this volume probably shouldn't have been APFS -- put that down to operator error. I will reformat as HFS+ if necessary. The volume wasn't backed up, but that's because there's nothing important on it that I can't reconstruct with a few hours work. However, I'd rather spend those few hours understanding the problem, if that's possible. I'm still puzzled why a volume that passes fsck_apfs with no errors can't be mounted, and why encryption doesn't resume.