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In general terms, it is safe to say that the error may be distilled down into the one specific items that indicates the error type or status, nearest the beginning of whatever error(s) the process is producing.
In this case, a '503' error simply means, 'the web service at the URL is responding minimally, but is Down, whether for maintenance or other reason'.
If the page URLs referenced are for Java servlet responder processes, which they seem to be owing to all the Java code referents in the error wodge you have posted... then Java servlet function would seem to be blocked at the server level, perhaps.
A thorough basic check - locally as to whether there is a JRE available (JRE=Java Runtime Environment), and at the server end, as to whether the site is down for maintenance by the web hosting co., or other reason/status - is advised.