After googling a bit on how custom classes were supposed to work on IW5, I ended up getting my answers.

So, on Steam IW5, the command

gameOpt commonOption.allowCustomClasses "1"

allows the clients to use private match custom classes (wich have everything unlocked) on dedicated servers.

Since we have ranked dedis, that command allow clients to use matchmaking custom classes instead (and that switching explains how the bug while choosing a custom class on a private match started). (*1)

Besides the fact that now dedicated server hosts can't allow clients to use those custom classes with everything unlocked, we now face a problem with our modders not being able to test stuff because they don't have stuff unlocked and they can't level-hack (my IW5M xlrstats template is stuck because I can't get log outputs of the weapons and killstreaks that I didn't unlock yet, although I keep headshooting my bot during the experiments in a desperate attempt to level up).

Although I don't have an idea how easy or complicated that might be, I'm gonna leave a suggestion here.

gameOpt commonOption.allowCustomClasses "1" // [ 0, 1, 2 ]

0= no custom classes

1= matchmaking custom classes like we have now

2= private match custom classes (with everything unlocked)

 

(*1) When you start a private match, the custom classes that show up are the ones that you made for matchmaking/dedis. But after you choose one of those classes, the class that you play with is actually the one that you created for private matches.

it'd not exactly be possible to switch the client UI to show private match classes, so such a setting might confuse users. I could add a dvar to disable script switching on the server however, so it'd act like a private match

As long as the server can use all IW5M features and allow clients to use those custom classes with everything unlocked...