If you are having problems loading in the server with the following error, here is the guide to fix it. This error is due to an update of GRAD Sling Helmet on the 24 JUN 2021 where the author renamed the files and Arma has sometimes forgot to delete the old ones.
You can not fix this bug by repairing/deleting the mod (no clue why).
HOW TO FIX THE PROBLEM
1. Find GRAD Sling Helmet in your Arma 3 Launcher
2. Do a Right Click and click Open Folder
3. Get in the folder
4. Go to Addons
And make sure you only have two files named :
if you have a file called "grad_slingHelmet" instead of "grad_slingHelmet_main", delete it, don't be afraid. Same for the bisign file.
5. Go back once, and go the Keys folder
And make sure you have only one file, not two, not three, ONE SINGLE FILE (with the following name or HIGHER version) :
6. Now, you can just go back to the launcher and hit PLAY
If it does not fix your problem, send me a message via Discord. Special thanks to @SPC T.Pearson for helping me on this and providing me the screenshots in english.
Corps of Engineers.