The administrative endpoint to reload named services (at .../profile/admin/reload-service) requires an id query parameter. If you omit the id parameter, or set its value as empty (i.e., ?id=) then you get a "Service not found." response as expected back via HTTP but in addition the IdP starts looping with the following message appearing in the logs infinitely repeated:
I'm guessing that this is because the error condition is handled by writing a complete response (line 104 of net.shibboleth.idp.profile.impl.ReloadServiceConfiguration), which commits the response and means that any subsequent attempt to do so will receive the IllegalStateException:
My SWF-fu is insufficient to guess where that subsequent attempt might be or why the looping is occurring. I find it interesting that this isn't signalled using an event, though.