Time Machine not backing up because it says FileVault is active, but FileVault is turned off

I have an iMac 21.5 (late 2013) running macOS 10:14.4. The FireVault screen (under Security & Privacy) says "FireVault is turned off for the disk 'Macintosh HD." and the "Turn On FireVault..." option is greyed out. However, when I try to use Time Machine to do a backup (even on a new and different external hard drive) an error message appears saying "Time Machine did not back up because FileVault was encrypting or decrypting a disk. Backups will resume once FireVault finishes." Since FileVault is not on, this doesn't make sense. I've run Apple Diagnostics and no problems were found. Despite searching the internet for an answer to this problem, I've not been able to find one directly on point and am at an impasse. Needless to say, I'd greatly appreciate any suggestions on how to get Time Machine to again do a backup.