I think I may see the problem...
Try connecting to the web admin and see if your players also get disconnected (I suspect they will).
I found with my P3 866 server, when I bring up the web admin or when a player brings up mapvote, I can't have more than about 150 maps in the maps folder or people get disconnected. It doesn't appear to have anything to do with Mapvote or the Web Admin, but with the code UT uses to catalog the maps (or any folder with a lot of files in it).
It seems that when folders with a lot of files in them are accessed by UT Script. It causes UT to temporarily stop transmitting packets to players, until it finishes accessing the folder. The problem seems to be logarithmic, with each new file added the problem gets exponentially worse.
I suspect (and could be wrong) that in a LAN game there may not be as much tolerance for slow connections as there is in an Internet game. As such, if the UT Server temporarily stops sending packets, you may loose connection faster in a LAN game than you would in an Internet game.
If you have a CPU monitoring software on your Server (TaskMan.exe, or TaskMgr.exe on MS Windows machines), try running it when one of your players attempts to bring up MapVote. On my servers (when I had over 150 or so maps) the CPU pegged out to 100% for several seconds. It did the same thing when I brought up the web admin interface. During the time the CPU was pegged, the network traffic from the server to the players stopped.
This is from your server log file.
Add mutator BDBMapVote304.BDBMapVote
Detected GameType = Botpack.DeathMatchPlus
Total Maps = 218
Try moving about half of those maps to temporary folder and see if the problem goes away.