I get that error still all the time. Yes I have tried re-starting and I have all 5 add-ons.
ORIGINAL: jaafit
I just joined a multiplayer session at a G3 airport and I'm not seeing these problems. Did you try restarting G3?
Also, there may be some messages that pop up telling you that certain objects couldn't be loaded. That's because the host has an add-on that you do not, and has created and airport using objects from that add-on. The airport will still load but without those objects.
Let me know if you're still having problems.