Could be an oxidation layer which have built up within the plug on one of the contact rings. Loud sound means more voltage and at some point it “breaks through” …
Shouldn’t be too hard to try to clean that.
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SO, I just opened it up again, and pushed gently on the headphone socket pins and tested. Works fine even when I left it for a couple of hours. So maybe just needs push/wiggle.
On further inspection, there’s a little bit of play in the 3rd connector when pushing in the headphone jack.
Pushing on the connector seems to have fixed it! 
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Unfortunately I don’t get any sound at all thru the headphone jack 
Stupid question: have you checked the mixer page? Maybe the headphones are set there to listen to cue out only …
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Just came to post about a single issue and myself thought it weird that the fix was to get a loud sound through, like hosing out a blockage in a pipe @tnussb 's explanation makes sense I guess, but it’s kind of weird in the experience of it…