Thank you all for the suggestions.
The power rails all measure good (unfortunately). I've tried cleaning the U400 contacts to no avail. Looks like it's the hybrid itself, as you have all said.
Unfortunately, I've got no spare 2465x lying around. Anyone selling a U400? :)
If I purchase one from the usual location (ebay), are there any differences between U400s pulled from different versions of the 2465/2465A/2465B, for example bandwidth. Or are they all identical.