Seems to be a problem with the server. Checked in clan chat and everyone using mafia was having the same problem. I'm using KoLmafia 15.4 r11443 which I downloaded this morning.
Server Error 5xx ... 10.5
500 Internal Server Error ... 10.5.1
501 Not Implemented ... 10.5.2
502 Bad Gateway ... 10.5.3
503 Service Unavailable ... 10.5.4
504 Gateway Timeout ... 10.5.5
505 HTTP Version Not Supported ... 10.5.6
If KoLmafia gets a 5xx as a response code, that is a server problem; KoLmafia did not submit anything incorrect (which would have resulted in 4xx codes) and there is nothing that KoLmafia can do to work around the server problem.
In some cases, however, it's possible that bogus input data would produce a 500 or 503, depending on how the server is configured to handle that data (ie. the server has an issue with the data you gave it). Not in this case though, as everyone else has already pointed out.