How can one debug imagent issues?
I removed the file

/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist


and after a reboot the error

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


was gone (Mac OS X creates a new 'NetworkInterfaces.plist' file during the boot process). 

The file 

/Library/Preferences/SystemConfiguration/NetworkInterfaces.plist


contains informations about the network interfaces, it looks like the upgrade from 10.7 to 10.8 broke something (at least on my system).

'imagent' itself has no debug option but you can try tools like 

dtruss 


or the

Instruments.app


to monitor the program. Instruments.app is part of XCode and a GUI to dtruss.

Or you can monitor the network traffic between imagent and the server.