It's a server(-side) code. And one of the absolutely most generic ones, that can't even be blamed on the browser getting the wrong idea (or told the wrong sort of things to ask for).
I'm used to getting 503 on another server, i.e. Service Unavailable (probably due to overloading/insufficient resources to handle the request).
500 is... a bit more vague than that. And generally nothing to do with your connection. (Unless that connection somehow involves a proxy server (that doesn't 502/whatever) that's playing it's own silly games whilst acting as intermediary.)
Noting that I've not been getting any errors at all, myself... (<knocks on wood for the immediate future>) Though obviously it is a concern (or atthe very least a matter of intrigue) to hear that anyone else has been. But also not likely to be something anyone other than Tarn (or 'his server people') can get to the real bottom of, I imagine.
edit: Ok. Just got one myself. Maybe two, but not sure if the browser actually attempted the first reload I tried, after a decent pause. But a(nother) pause later got a normal response and no followup issues up to the point of wandering back here just now. Definitely server-side. Definitely lacking any useful user-side info. If there's anything in the server-logs (or otherwise detectable rackside), then I wouldn't know. Let alone anything automatically pinged up to the respective admins/Tarn to forewarn them of potential user discontent...