Have you changed anything or unplugged or re-plugged anything since this happened? If so, the usual culprit is getting the DIN-XLR leads the wrong way around (correct is Red band on CH2 - Right, Green band on CH1 - Left).
Otherwise it could be a sticky relay. A power cycle or two may unstick it. Otherwise it’s a repair/replacement scenario.
A dodgy solder joint on an interconnect can also be responsible for this sort of problem. A continuity check of the DIN leads can rule those sorts of problems out. Your diagram looks ok to me.
I had something similar on my 82. The advice on the forum was to press and release the mono button a few times and see if that worked. It DID work. I pressed it fewer than 10 times. I wouldn’t know if that would also work on a 252 . The relay was the other suggestion.
Yes … I should probably check the Burndy from 252 to SuperCap … I assume these are wired A-A, B-B etc…
All other cable have been swapped out, so should be ok…