This might address the following observed exception:

17:14:28 - [APPLICATION]:
APPLICATION EXCEPTION DETECTED: System.UnhandledExceptionEventArgs
Exception: System.InvalidOperationException: Operation is not valid due to the current state of the object
  at System.Collections.Generic.Queue`1[OpenSim.Region.ClientStack.Linden.WebFetchInvDescModule+aPollRequest].Peek () [0x00011] in /root/install/mono-3.1.0/mono/mcs/class/System/System.Collections.Generic/Queue.cs:158
  at System.Collections.Generic.Queue`1[OpenSim.Region.ClientStack.Linden.WebFetchInvDescModule+aPollRequest].Dequeue () [0x00000] in /root/install/mono-3.1.0/mono/mcs/class/System/System.Collections.Generic/Queue.cs:140
  at OpenSim.Framework.DoubleQueue`1[OpenSim.Region.ClientStack.Linden.WebFetchInvDescModule+aPollRequest].Dequeue (TimeSpan wait, OpenSim.Region.ClientStack.Linden.aPollRequest& res) [0x0004e] in /home/avacon/opensim_2013-07-14/OpenSim/Framework/Util.cs:2297