[Bigbuck]

If I remember correctly, some were having that issue because of the SM nextmap.smx plugin. That plugin won't work with L4D and should be disabled by SM, but it can't hurt to move it to the disabled folder. If that doesn't work I would try it on a fresh install of the L4D server and see what happens. If you could post your server.cfg, minus the rcon info etc, and a list of all the plugins installed maybe we can narrow it down.