OK, found the
topic here ==8<==============
A piece of advice in order to design coop missions for the 2.51 version :
1/ do not use the SAM external view, F7 key
2/ do not jettison weapons nor tanks, key ctrl w
3/ do not include IA flights with a loadout that they can jettison : namely fuel tanks, bombs and air-to-ground -or sea- missiles. BUT, fortunately, this is not valid for loadouts in internal bays, in the heavy bombers like the Tu22, B52 or even in the F117, because the IA does not jettison its weapons in internal bays. Hence, if you want to use some IA flights in an air-to-ground role, those IA flights can be fitted with rockets, or other weapons only in internal bays.
4/ do not use S13, S24 and S25 rockets against active SAMS
If the rules 1/ to 4/ are not respected, all the clients will experiment an "internal error" at the very moment when something forbidden, like jettisonning, happens.
5/ do not use composite SAM -that is, SAM with a separated detection radar, track radar, and launcher like the S300, Buk, Kub, Patriot, Hawk ...-. They can prevent the clients from connecting, or can produce clients disconexions during the mission. It might be possible to use those composite SAMs, above all if they are to be "treated" by SEAD flights before other aircrafts arrive ... but do this at your own risk ... 2.51 is buggy enough to add those "risks" IMHO. On the contrary, ships with long range SAMs do not cause problems.
6/ do not include IA flights with more than 2 aircrafts
7/ do not include a composite IA Airgroup, with a Ground Attack and an Escort for instance in the same Airgroup.
6/ y 7/ can produce a strange effect of IA lagging or disappearing, wich is very annoying when you are fighting in BVR agaisnt those super stealth IA
8/ for human flights beginning from the runway, do not use the "0" position if you plan to record a track, because the track won't play back correctly.
Well, I think that's all concerning the specifics bugs of the 2.51 version. But on top of that, you also have to take the same precautions than with the 2.03, such as not using to much IA, SAMS or flights, at the same time and the same area.