To start off with, me and my friend play with a massive modpack with mods that add a variety of nationalities. As such I have close to or over 100 loadouts saved. Everything seems to work fine with our modpack, however my friend has experienced this problem in the past even with a lot of mods installed.
I noticed the problem when we tried to play Invade and Annex on a vanilla server. I had already made a loadout in vanilla Arma for situations like this where we take a break from our modpack to play on a public server. I got on the server and then it wouldn't allow me to load it. I figured I'd go into the virtual arsenal and check what I have on the loadout that might be considered OPFOR or something that might be blocked. Once again it wouldn't load. I restarted my game and it wouldn't load still. I then realized there was a random pattern to which loadouts would let me load and which wouldn't, which was the exact same as when my friend had this problem. His fix was he deleted a ton of loadouts. Suddenly he was able to load all the ones he hadn't deleted. I haven't tried it myself because I actually need mine.
Here's a video demonstrating the issue: https://www.youtube.com/watch?v=g8Er8yTkmfE