1- Everytime you start the game, open the in-game console and type "sv_consistency 0" into it, then press enter. This makes you be able to player that have different addons than yours.

2- There's a ton of new mods that have been released after this one, and surely some of them may cause issues, if you install an addon and this one start to malfunction, make sure to uninstall the mod that you just downloaded in order to be able to play.

3- Watchout for mods that change scripts, such as vocalizers, spawn, bots ai, ui, huds, effects, voices and some skins mods. It may help not only avoiding issues in this addons, but aswell in various other addons.

4- If the menu simply stops working, without you installing nothing new, check if any of your addons have been recently updated, and if it does not, do the step bellow

5- If the addon stops working for no reason, check the steps above. If none of them pointed the issue, try reinstalling the game and disable all addons you have installed, also be sure to delete addons stored in your addon's folder (usually the path is: C:\Program Files (x86)[if you're using a 64bit system]\Steam\steamapps\common\Left 4 Dead 2\left4dead2\addons\workshop). After deleting all the addons, download only admin system and admin menu and check if still has issues, then install the other mods you have 1 by one until you find which one is causing the issue.

6- Make sure that your steam is updated and your game is working fine, if it isn't, try to reinstall it in another place and check if it fixes the issues.

(Note that this mods needs of admin system in order to work, if something is causing incompatibility or any other issues with it, it will cause malfunction in both admin system and admin menu).