if you get this error, it should be for various reasons, the most common one is:
I am doing some big changes in tables/database, so myisam mysql table is locked, that means web requests are halted, then overflowed and then you get http status code 503. Another reason can be, something else is wrong.
Make sure you go to
www.opensubtitles.org or api.opensubtitles.org/xml-rpc and see if it is working (200 OK)
I hope this helps.