HP34401A - error 608

[Tlmadsen]

My trusted HP34401A seems to have developed an error I am strugling a little with. It simply don’t measure anything more and running the full self-test it comes up with error-608: Serial configuration readback failed. Seems like the “chain” to the U501 is somehow broken, but is is not clear to me how ?. I changed the PSU-caps, but as expected it didn’t solve the problem. Inputs are appreciated
Cheers
Thomas