I have configured SQL Server session state. At first it works as expected - I 
can trace several calls through the code to SQL. After about a minute the 
application fails with "Invalid token for impersonation - it cannot be 
duplicated".

Anybody knows what's going on here?

Type : System.ArgumentException, mscorlib, Version=2.0.0.0, Culture=neutral, 
PublicKeyToken=b77a5c561934e089
Message : Invalid token for impersonation - it cannot be duplicated.
Stack Trace :    at 
System.Security.Principal.WindowsIdentity.CreateFromToken(IntPtr userToken)
	   at System.Security.Principal.WindowsIdentity..ctor(SerializationInfo info)
	   at System.Security.Principal.WindowsIdentity..ctor(SerializationInfo 
info, StreamingContext context)