Same problem since on the 4th of April installing the latest Mojave upgrade. Tried everything mentioned above and on some other threads, including the "Reset of NVRAM or PRAM" — and nothing has worked. After that reset and after previously turning off the Mac and unplugging it several times, I got one finished backup each time then the same problem and message (Time Machine did not back up because FileVault was encrypting or decrypting a disk. Backups will resume once FileVault finishes). Never turned on FireVault, and checked in the Activity Monitor that is was off.
I have three external backup drives with my desktop. Two are connected to the USB3 ports, one to a hub. I've run the Disc Utility First Aid on all, and on the computer's internal HD, and all checked out.
Maybe replacing the interior battery is the last step?