This is a quick one and one I am going to jot down because sometimes the resolution to issues is really quite simple.
Last night, I deployed SSPR onto its own server, and in my validation of the implementation, each time I accessed one of the SSPR portals I would get a 503 error (i.e. service unavailable) with no corresponding event entry getting logged. Stepping through it, I notice that the app pool services were running, but when I would hit either the registration or reset portals, the services would stop. Weird eh?
Weird yes, but simple to fix. Check to make sure that the password for the app pool service account is correct. Reset it in AD and/or reset it in IIS Manager to bring them in-line. $10.00 says that this fixes the issue.
Hope this helps if you have this too