WTF IS HAPPENING?
Fix partnering.

FIX IT.
FIT IT.
FIX IT.

For some reason, the currentUser can set the "partner" column for their partner... but cannot create an entry for their partner in their OWN "partner" column. No idea why.
Perhaps PFUser's are only capable of creating pointers of themselves?

So. You can either stop trying to store the partner within the User's data, and just use the Friendships table... which already exists and functions.
Or you can struggle with this crap for six more hours and waste a bunch of time on it. Your choice. One of these options isn't stupid. Up to you... Do the first one. Please.