The WW4_Weaponry error is something i noticed several months ago when playing some multiplayer missions.
I found the source of the problem : when creating and saving a mission in multiplayer, the WW4_Weaponry class was not saved in the mission.sqm addon list for reason i never understood (as everything seemed to be correctly defined in configs.
I fixed this problem by making the WW4_Weaponry (it the cfgpatch name of the ww4_magazines.pbo own config.cpp) to be preloaded by OFP/CWA (it's listed in the config.cpp preload addon section).
Since fixing it, in further mp games this message never appeared anymore for any of us.
I'm really curious as to why this is happening for you.
Because with WW4_Weaponry being preloaded it should not be possible that it being listed as missing from a mission.sqm addon list would trigger the message again.
As i notice you're using more than ww4, are you using a different main config.cpp that would come with one of the addon pack you're using along WW4 ?
Or a custom config that would be missing the WW4_Weaponry entry in the addon preload section of the config ?
in the end of the main modified config.cpp you may use for ww4+whatever other mod running, be sure you have :
Does this happen on a specific mission ? (and then can someone upload it so i can check the mission.sqm to see if i can track the source of that new problem ?)