I tested removing the briefing, mission name and pictures yesterday.
It did make a certain mission appear and we could connect to it but, there were duplicate connections done and various clients got CTDs.
I'm guessing it is part of the issue but not the whole issue.
I've went through the mission file, the only changes I found were the coalitions in the old converted missions did not have neutral coalition while new mission did.
I tried to rectify it manually but this unfortunately was still not enough to fix it.
I hope ED gets a fix on this.
EDIT - We have found that removing the briefing plus a combination of some modules and then saving, will solve the issue.
Once we have a complete fix on how to workaround this we will update here.