Party Member Preprocessing

With the change of each map/mode I am stuck in a ever lasting load reading "Party member preprocessing" with the only, temporary, soultion being to close & re-launch or back out to the main menu then join back in every time.

This problem also effects my friends as they can not join me nor can I join them. 

Side-note; interestingly when my US friend joins my EU server the problem goes away.

Does anyone have a fix?

Thanks