[Bugsy]

I am not having an issue, it seems to work normally for me but I only gave it a quick test. You aren't doing any reference to entity index so I don't think it is because of that. There may be other things that are wrong in your code but the below are the things that I quickly noticed.

public addkey( id, level, cid )
{
    if( !cmd_access( id, level, cid, 2 ) )
        return PLUGIN_HANDLED;

    new Arg1[16];
    read_argv( 1 , Arg1, charsmax( Arg1 ) );

    new adder[64];
    formatex( adder, charsmax( adder ), "models/w_%s.mdl", Arg1 );

    new szVaultData[ 60 ] , iTS;
    
    if( nvault_lookup( gNewVault, adder , szVaultData, charsmax( szVaultData ) , iTS ) )
    {
        client_print( id, 2, "This key is already saved in the pick up manager list." );
        return PLUGIN_HANDLED;
    }

    //This loop should begin at 1 since item 0 is "========== All weapons-key avaible to add to add in list ========== ",
    for( new i = 0; i < sizeof ( gKeyList ) ; i++ )
    {
        //You should search the entire list before deciding whether or not the key exists in the list. In your code, the loop is going to
        //exit after the first iteration.
        
        //contain/containi/strfind all return the position of the found substring. In some cases, this can be at the beginning of the 
        //string which is position 0 so doing a !containi() condition is wrong, it should be if ( containi() == -1 ) then string not found.
        if ( !containi( gKeyList[i], Arg1 ) )
        {
            client_print( id , 2, "Unnable to found this key, type amx_cspum_keylist for all keys avaible to insert." );
            return PLUGIN_HANDLED;
        }
        else
        {   
            client_print( id , 2, "You succesfully added %s in list!", Arg1 );
            nvault_set( gNewVault, adder , "1" );
            return PLUGIN_HANDLED;
        }
    }
    
    return PLUGIN_HANDLED;
}

public OnPlayerTouchWeaponBox( ent , id ) 
{ 
    if( !ent || !id )
        return -1;

    new szWeaponModel[40];
    pev( ent, pev_model, szWeaponModel, charsmax( szWeaponModel ) )

    new iCvarValue = get_pcvar_num( giTypeCvar );

    new szVaultData[ 60 ] , iTS;
    
    //I would not call nvault_lookup() in a touch forward. This is called way too much and the nvault module is getting 
    //spammed with lookup calls. You should store each of your weapons status in an array, maybe refresh it each round.
    if( nvault_lookup( gNewVault, szWeaponModel , szVaultData, charsmax( szVaultData ) , iTS ) )
    {
        switch( iCvarValue )
        {
            case 1: engfunc( EngFunc_RemoveEntity, ent );
            case 2: return PLUGIN_HANDLED;
            default: return PLUGIN_CONTINUE;
        }
    }

    return PLUGIN_CONTINUE;
}