I think what may have happened is, during an upgrade or downgrade, your custom keys were deleted from the plugin's saved options. The custom meta keys still exist in the database, but do not exist in User Meta Manager's saved data. So, when you attempt to recreate the key, it fails because the key DOES already exist in the database. And, when you attempt to edit the key, it fails because the key is no longer in the plugin's saved data.
The custom key exists in one area but not the other. It must exist in both database AND User Meta Manager saved data. The data is out of sync.
So what we need to do is copy the existing custom keys over to the new data structure, or delete the existing custom keys from the database and start fresh.
I hope that makes sense. :)