I did restore the config file, but it didn't help. I tried several methods to turn on error logging via the http service and iis. None of those worked (error logs didn't show up in the designated locations). There is nothing in the event log. I changed service accounts for SSRS several times. I deleted the web sites, added them back, several times. I made various recommended edits to the reports config, then undid them when nothing changed.

Overall, about 10 hours spent, nothing has changed. I don't know what's wrong and I don't know how to fix it. Various people online have fixed their 503 errors with wildly varying solutions, indicating that several underlying conditions cause this. Without knowing what this one is, I don't have any idea how to proceed.

My mext step is to start uninstalling stuff and reinstalling. This increases my risk of "breaking more stuff" but I don't see any alternative now.