PfSense 2.1-RELEASE (Hyper-V) crash "Panic String: sleeping thread"

[mortem]

Hi All,

We setup 2 pfSense for 2 different offices using the same settings as below:
version pfSense 2.1-RELEASE (amd64)
Platform: Hyper-V virtual machine with 4 CPU, 4 GB of RAM, 2 NIC and 40 GB of HDD
Packages: squid3, squidguard, lightsquid

Since the systems are in production, one seems crashing every 24 hours only while the other one crashes about every 15 minutes.
Before being in production (a few days long), not a single crash occurred.
They both have few load: about 5000 states, 10% MBUF usage, CPU usage 5%, memory usage 15%, etc…

The Crash report looks like that:
Crash report begins.  Anonymous machine information:

amd64
8.3-RELEASE-p11
FreeBSD 8.3-RELEASE-p11 #0: Sun Sep 15 12:27:10 PDT 2013    root@fbsd83minx64.corp.itbxb.com:/usr/obj.pfSense/usr/pfSensesrc/src/sys/pfSense_SMP.8

Crash report details:

Filename: /var/crash/bounds
71

Filename: /var/crash/info.0
Dump header from device /dev/da0s1b
  Architecture: amd64
  Architecture Version: 1
  Dump Length: 71680B (0 MB)
  Blocksize: 512
  Dumptime: Sat Jan 18 09:05:35 2014
  Hostname: server_hostname
  Magic: FreeBSD Text Dump
  Version String: FreeBSD 8.3-RELEASE-p11 #0: Sun Sep 15 12:27:10 PDT 2013
    root@fbsd83minx64.corp.itbxb.com:/usr/obj.pfSense/usr/pfSensesrc/src/sys/pfSense_SMP.8
  Panic String: sleeping thread
  Dump Parity: 982509593
  Bounds: 0
  Dump Status: good

Did anyone found a way to solve that issue? I checked on the forum but didn't find any solution :(

Thanks a lot

PM