I'm using MM5 to upload songs to google play music, and when I tried to do that today, I was greeted with a MAX_LIMIT_REACHED error.
Turns out that gpm has a hard limit of 10 devices per account and I have reached that limit (and for whatever reason you can only deauthorize 4 devices per year).
I only access gmp with MM5 on a single computer (except for two devices which are explicitly listed), and the timestamps match dates for which I know I used MM5 to upload songs to gpm, so it's probably a safe bet that MediaMonkey is related to the issue in some way or another.
I haven't checked how the authentication with gpm works and if the authenticating app needs to provide some kind of device ID which is used for the device limit. If that's the case, it might be a factor that I usually keep up with new MM5 releases.
I install MM5 portable, so when upgrading to a new release I back up the portable folder, remove the old MM5 directory, install the latest revision and move the backed up portable folder to the new directory. Data that's persisted in the database or persistent.json should therefore carry over, even between releases.
Update: To further test this, I uploaded some songs to gpm with MM5 yesterday evening and retried the same today:
Both uploads were done with the same instance of MM5, no settings were changed, and I made sure not to access gpm with any other device or application.