ajax response is 503

ajax response is 503

Hello,
I am trying to send data of form using $.ajax function (especially I am doing chat for phpBB3), but while posting non UTF-8 characters, everything is ok, however, if I will send "+ľščťžýáíé=", it will lead me to status = 503, and I am not sending it in script. Is possible that this is bug, or I do not know how to send UTF-8 characters, but I tried anything I found on the internet: async, dataType, contentType; but nothing helped me.

Thanks in advance for any reply!