hey ive installed the release 0.5.9 on debian

if i start opensim it tells me this error

** (OpenSim.exe:24249): WARNING **: Missing method System.Net.ServicePointManage                                                                                                 r::get_ServerCertificateValidationCallback() in assembly /usr/lib/mono/gac/Syste                                                                                                 m/2.0.0.0__b77a5c561934e089/System.dll, referenced in assembly /root/opensim/rel                                                                                                 ease/opensim-0.5.9/bin/OpenSim.Region.Environment.dll
[APPLICATION]:
APPLICATION EXCEPTION DETECTED: System.UnhandledExceptionEventArgs

Exception: System.MissingMethodException: Method not found: 'System.Net.ServiceP                                                                                                 ointManager.get_ServerCertificateValidationCallback'.
  at <0x00000> <unknown method>
  at OpenSim.Region.Environment.ModuleLoader.InitialiseSharedModules (OpenSim.Re                                                                                                 gion.Environment.Scenes.Scene scene) [0x00000]
  at OpenSim.OpenSimBase.CreateRegion (OpenSim.Framework.RegionInfo regionInfo,                                                                                                  Boolean portadd_flag, Boolean do_post_init) [0x00000]
  at OpenSim.OpenSimBase.CreateRegion (OpenSim.Framework.RegionInfo regionInfo,                                                                                                  Boolean portadd_flag) [0x00000]
  at OpenSim.ApplicationPlugins.LoadRegions.LoadRegionsPlugin.Initialise (OpenSi                                                                                                 m.OpenSimBase openSim) [0x00000]
  at OpenSim.ApplicationPluginInitialiser.Initialise (IPlugin plugin) [0x00000]
  at OpenSim.Framework.PluginLoader`1[OpenSim.IApplicationPlugin].Load () [0x000                                                                                                 00]
  at OpenSim.Framework.PluginLoader`1[OpenSim.IApplicationPlugin].Load (System.S                                                                                                 tring ) [0x00000]
  at OpenSim.OpenSimBase.LoadPlugins () [0x00000]
  at OpenSim.OpenSimBase.Startup () [0x00000]
  at OpenSim.OpenSim.Startup () [0x00000]
  at OpenSim.Application.Main (System.String[] args) [0x00000]

Application is terminating: True