If having the box "not there" solves the problem, it should be because of the SCART control voltage from it coming and going for some reason, and therefore confusing the MX.
Then you can try disconnecting pin 8 from the SCART lead (or as a reversible fix, wrap a piece of hard tape over pin 8 in the SCART plug to isolate it, it should last a couple of insertions). The MX doesn't support aspect ratio control anyway so the signal isn't really needed for anything.