I am getting the same problem. I have tried to join an instanced group through the duty finder and after approximately 15 seconds (long before the q would actually be filled) I receive the message. I also tried joining an instance in a premade group and received the error message.

The message is "Unable to connect to Duty Finder server. Please try again later."

I logged off long before the emergency maintenance started last night, and did not try to log in until after it was completed, but I have to assume this problem is somehow in conjunction with it.

I tried submitting a "report a bug" ticket, but I got a very quick no answer response that does not help me at all.