Before you go any further, let me just remind you that using LuaSocket is a bad idea. It uses blocking socket calls, which means the server won't be able to continue until a network event occurs; this means that the server's world tick threads will stall within your plugin, as well as other threads. Although it is possible to use LuaSocket in non-blocking mode, I'm not sure if the http library supports that and it becomes a nightmare to program.
The other Lua scripts on the same machine, do they use Lua 5.1 or Lua 5.2? We use Lua 5.1 in MCServer, so any libraries installed for Lua 5.2 might be incompatible (see the "5.1" in the listed paths). I did manage to install LuaSocket via LuaRocks on my virtualboxed Ubuntu and it worked alright with MCS, so I suppose there's some hidden misconfiguration that we need to find out about.
The other Lua scripts on the same machine, do they use Lua 5.1 or Lua 5.2? We use Lua 5.1 in MCServer, so any libraries installed for Lua 5.2 might be incompatible (see the "5.1" in the listed paths). I did manage to install LuaSocket via LuaRocks on my virtualboxed Ubuntu and it worked alright with MCS, so I suppose there's some hidden misconfiguration that we need to find out about.