swap the cans.
if the problem moves : can problem.
does the relay click if you change ac/dc ?
they can get stuck .... they have mechanical memory ( they are latching relays )
also check that the center pin of the bnc is not broken off !
I have three mode 1M DC, 1M AC and 50 ohm DC.
If you change between 1M DC and 1M AC no click, but if you change to 50 ohm DC you can clearly hear the click
and you can hear it again if you switch back to 1M AC.
However thank you, because now I have some clues of what the problem is.
When I'll find the time to take it apart I will try to exchange the cans and I will update
the thread.