Quite a lot of people has been having this problem so we've had a server restart today, might've solved it for you as well.
This happens a lot when users run in and out of custom maps causing the server process for it to start, stop, start, stop, start, stop, which eventually could lead to "failed to connect to map server", and the only way for it to start working again is if we schedule a server restart. It's more likely to happen to active/popular custom maps since a lot of people tend to go into them, but not enough to always keep 1 user in there at the time, because as soon as no users are in a map, it stops.