Unable to mount obb file created with JOBB

Question

I'm trying to mount either an encrypted or non-encrypted obb file I created with the JOBB tool using the mountExpansion() method found in this question: Mounting an encrypted obb apk expansion file in Android. What is interesting is that when I try this with an encrypted file, I get the state 21 which according to https://developer.android.com/reference/android/os/storage/OnObbStateChangeListener.html means

The OBB could not be mounted by the system. Returned in status messages from calls made via StorageManager

and when I try a non-encrypted file I get state 20 which means:

There was an internal system error encountered while trying to mount the OBB. Returned in status messages from calls made via StorageManager

Neither of these explanations are very helpful in what might cause the states to occur. Any suggestions for this?

The comments in an answer to a similar issue I had may also be relevant: https://stackoverflow.com/a/37957811/1489990 as the only way I was able to create the obb file with jobb to begin with was to force FAT32 to be used instead of FAT16.