We are seeing an unusual amount of 503 errors on public reverse proxies, we are looking into it.
EDIT 21:28 UTC: The issue has been found and fixed. We are monitoring the situation.
EDIT 21:40 UTC: Everything seems to be back to normal. The issue was happening for a couple of applications starting around 16:30 UTC. We will investigate further on why its configuration was out of sync during that time period.