[Bacardi]

Do you have now any mods installed on server ?
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If you have mods, you could try temporary disable MM:S and SM from server, just to check problem is not there.

Find metamod.vdf file from ...left4dead2/addons/ folder, move that file out from ...addons/ folder. Reboot server.

Now you should not see mods loaded on server
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-Then try get person to connect to server.

When you have tested and want mods back on server, move metamod.vdf file back in ...addons/ folder, reboot server.