How to debug imagent?

I have just upgraded to OS X 10.8 Mountain Lion and am attempting to use Facetime and iMessage. Upon entering my Apple ID into the respective applications and attempting to sign in, I receive the following error message:

The server encountered an error processing registration. Please try again later.

Whilst I assume that imagent proceeded beyond the authentication stage (as providing incorrect credentials results in a different error message), I am unable to ascertain exactly what went wrong. I cannot find any log file (nor any option to enable logging) that shines light on the subject. Is it possible to auditimagent activity?

I have found (mostly on these forums) various suggestions, some of which appear to have worked for others:

1. Force quit the imagent process;
2. Reset /etc/hosts to default state;
3. Enable ntp;
4. Fix permissions and/or delete ~/Library/Preferences/com.apple.imagent.plist;
5. Delete certain Apple-issued certificates from the user's login keychain;
6. Use Google's public DNS resolvers;
7. Reboot.

I have tried all of the above, with no improvement.

I have tried using two different Apple IDs: one a simple MobileMe account (ending @me.com) that has been transferred to iCloud; the other an old .Mac address (ending @mac.com) with which additional non-Apple email addresses have been associated and verified. The computer is connected to iCloud with the former account; and I have previously used the latter account successully with Facetime on the same computer (when running Lion) and with iMessage on my iPhone and iPad.

I am at a loss to understand how I should proceed. Whilst I would be grateful for any suggestion that fixes the problem, I would be far more grateful for a canoncial answer that describes how imagent issues can be debugged in the future: e.g. how to enable logging or similar.