Westfalia detachable tow-bar - stuck in place!

Wondering if anyone has any relevant experience.

My tow-bar has had very little use, never in the wet, still clean as new and with original grease visible in the mechanism. (Housing always had the stopper in it and was clean and unobstructed.)

Problems started last time I tried to mount it – it wouldn’t engage properly, the knurled wheel didn’t rotate the full extent, red marker stayed midway in the green sector, the wheel was still 5mm clear of the base and with barely any clock-wise pressure it would immediately dis-engage. I gave it umpteen attempts with the same result – about to give up in despair and then finally it locked into place, with the usual solid ‘clunk’. However I also definitely heard an unusual metallic ‘ping’ noise.

After towing, I tried to remove it – absolutely no-go! The key rotates fully and goes in and out, but the knurled wheel will not turn at all (it still pulls out the normal 5mm, but that makes no difference to anything).

So - its totally stuck in situ. Not the end of the world, I know, but annoying for something that, although five years old, is still in as-new condition. Before I try to look for a Westfalia dealer, has anyone had a similar experience or, if familiar with the breed, can make any useful suggestions?