The back panel connector SHOULD NOT ROTATE. That was probably the source of your problem. Once you get this fixed, tighten the nut and maybe put some hot melt glue on it to secure it from loosening. What you call "rubber housing" is probably plastic shrink wrap tubing intended to insulate the wire soldered to the connector terminal. You could also use some electrical tape for this purpose.
My Limex installation has the connector at the lower front corner of the bass panel, and oriented so the cord attaches parallel to the bottom of the accordion. The cord is run through the bottom of the bass strap. This way, it will not rotate as you move around. See photo.
Two suggestions, if you can't get the repair done yourself...
1. It should be possible to disconnect the connector cables from the Limex circuit boards. You could then ship only the back panel to Tim for repair. Of course, you will have to make sure you reconnect them correctly when it is returned.
2. Tim may be able to supply you with a new back panel connector assembly (DIN plug, attached cables, and capacitor) that should be easy for you to install after removing the old one. Maybe the best option if he has these available.