Encrypted APFS External Drive Not Unlocking

I have a 5 TB SanDisk Professional G-DRIVE ArmorATD external hard drive. I’ve been using it for about a year now, and suddenly I get the prompt that it can’t be unlocked because there is a problem with the disk.

I have it formatted as APFS, with one APFS (Encrypted) partition that holds just my normal data, an APFS (Case-sensitive, Encrypted) partition that holds Time Machine backups, and another Time Machine partition (Case-sensitive, not encrypted). See photo below for structure.

None of the portions will mount. I’ve tried to mount the partitions using Terminal using

diskutil ap unlockVolume diskname -noMount

but it still won’t unlock. I get this when I try:

Passphrase: (encryption key entered)
Unlocking any cryptographic user on APFS Volume diskname
Passphrase incorrect or the user does not exist. 

I know that there isn’t any physical damage to the disk, because I ran a hardware check on another Mac using Drive Genius (unless the check was wrong).

When I tried to run First Aid in Disk Utility on one of the unencrypted Time Machine backup partitions, it started to run, but said it failed to run. I tried again, but this time Disk Utility stopped responding. (See log below).

The strange part: when I try to get the files on my 5th-gen iPad Air, it mounts the encrypted non-Time Machine backup partition, and only shows some, not all, of the files on that partition.

Any help would be greatly appreciated, as I don’t want to spend hundreds of dollars to get a professional to recover my data.