Error: Element SYSTEM is undefined in INSTANCE - Transfer?

Ok - so this was a first time i'd ever seen this error and I was a little stumped. The error come when I tried to login to a site which I'd done a 'thousand' times before. Looking at the stack trace I could see the error was caused by Transfer. Hmmm ok, I've just updated transfer locally so was that it? I looked over the code (a parsed fusebox file) but I couldn't see anything different? It really had me stumped. Time to call in some help. Mark Mandel to the rescue. After sending Mark some details about the error he came back with a simple solution...

"Have you cleared out the Transfer Cache files since the upgrade?"

Doh! I'd cleared the fusebox parsed files when I did the Transfer upgrade but I hadn't cleared the transfer cached files. After doing this it worked fine...

The moral of the story: Clear cached files if anything doesn't appear to be working correctly before bothering people!

BTW: If you've not upgraded to yet it has a load of enhancements.