When do the players get this error? Is it as they connect to the server after about 15 players join, do they get insta kicked, or is it like, after 15 join, people start randomly getting kicked from time to time?
If it's the first:
Are you sure it's not the plugins causing the problem? All the stuff the server sends to the player; map, textures, plugins, models etc, can contributes to this error. If there's too much information being sent at once, you get this error.
I presume the error doesn't happen in a LAN game right? If that's the case, then all the stuff on the server is causing the overflow.
If it's the second:
You said that your map is below the entity limit, but remember that entity polys in-game increase based on how many players there are on the screen, so if you start getting overflow after about 15 players join, it could be due to too many entity polys. Of course plugins contribute to this as well, If the sever is hosting a map with a lot of e-polys, and it has flashy plugins that make models appear, create flashy sprite effects or what not, it would make sense why it chokes after a certain amount of players join.
To fix this, You can either reduce amount of entity polys for map, or remove some flashy plugins and see if it can handle more than 15 people now.
Does your server require specific rates? and if yes, are the players with those rates crashing? Reading this up on the web, it could be caused by bad rates in-game too, but then I guess it wouldn't make sense why it happens only in your map.
Give us the .rmf and .wad's so we can find the problem.