A completely server-side implementation of the replay mod, this mod allows you to record multiple players that are online on a server at a time, as well as any given chunk area. This will produce replay files which can then be used with the replay mod for rendering.
Please provide more details, including the mods your server has, the version of server replay, the version of minecraft, any logs etc.
Specifically do you have Very Many Players installed? In which case this will be closed as a duplicate, that issue has already been fixed.