What could cause this?

[Electronic M]

That silicone looks factory original to me. I've seen plenty of unopened electronic gear with it inside. It had various uses, preventing adjustments from being tampered with, preventing large (otherwise) poorly secured parts like main lytics from wiggling in transport, etc.

If I was in your shoes and happened to like it enough to bother I'd touch up and suspicious solder joints (and the ones on the high power stuff like PS and sweep sections), then hit the caps with an ESR meter to look for bad ones.
Maybe ~10% of my SS gear I like well enough to give that courtesy upon death/failure, the rest I scrap without a second thought....Life is too short to waste on garbage...If it proves obstinate chuck it, grab another off the free section of craigslist and move on with yur life.