How long the mission should be opening?
I'm waiting 2 mins and still nothing... just 'loading mission'
That is bad. The first load of mission file usually takes a longer time but not 2 minutes or more. Have your Windows regional settings switched to US/English? WOFF and ME recognize decimal points as dots "." instead of comma ",". It is possible your mission file contains commas that caused ME to hang.
Can you send me, pls, your MissionEditor.log file located at WOFF UE Mission editor folder and your OFF_Camp_Mission.xml file that caused the Mission Editor to hang? Let us continue over PM, pls.