The difficult-to-track-down Last.fm bug should be fixed. After hours of looking at debugging output, proxy logs, and packet dumps, it seems the Flash sandbox thought there was a security violation. What made it especially hard was that logging in always worked, so I assumed the bug wasn't with security. Basically http://any.domain.com is allowed, but http://22.214.171.124 isn't. Audioscrobbler provided me with a URL with an IP address.
Luckily, the URL doesn't seem to change. Reverse DNS gave a reasonable domain name, but it didn't actually resolve. I set a domain under a domain I control to resolve to that IP address. It makes sense that it's fixed, but that shouldn't be a fix.