Additional Instances Cannot be Launched

Me and my girlfriend returned to play WoW on Monday, since then we haven't been able to join a single instance because of this.

What is it and why is it happening?

I never had this problem when the game launched

I never had this problem when TBC came out

I never had this problem when WOTLK came out

Instancing is what made WoW popular and if people cannot get into an instance then why will they hang around?

FIX IT.