FAQ - Frequently Asked Questions & Answers thereof

Error 503 means "service unavailable". This can happen if a previous synchronisation attempt has recently failed. Apparently, the OCS server needs to time out the previous session before it can start a new one.

Just waiting about 5 minutes and then trying again usually helps.

Note that this implies that you should not set AutoSync interval to less than these 5 minutes, as otherwise you might be trapped in an endless loop of retries and Error 503.