that would have to be the reason, cause there is no limit. my friend has 127 plugins, so a limit is not the problem. at the top of the server it says something about "to start the server with more ram do this ..."or at least mine does, maybe see if that helps.
Which plugin are you trying to install? Have you tried with more than one plugin? It could be that the plugin you are trying to install is breaking it, and any other plugin would work fine
