There's a few possibilities and I'm short on time...
Is this just in multiplayer, program missions or all?
It's been a long, long time since I've engaged in multiplayer and remember no problems when I did (used only default acft then, as well). In regular play, however, if there is an eror in any of the add-on (actually, default, too) acft, the system will lock out when that file is finally accessed. This is also true for static objects not initially loaded until you reach that waypoint area where its files are accessed.
There are other possibilities, pending