PRTG failed to connect to network devices (SNMP error # -2006)

Dear support,

Previously PRTG works with no critical errors since I have it installed. Two weeks ago I had the same situation when PRTG cannot connect to any switch using SNMPv1 or any server running microsoft windows using WMI, HTTP and etc. (except ping, it reports Ok anytime on any device). This collision returns back today. Last time I restarted whole server with PRTG and problem disappeared.

As check result on any snmp sensor on the core-switch PRTG reports: Error opening snmp session: Unknown host (10.9.1.1:161) (SNMP error # -2006). I tried to run snmp tester from probe machine and it reports:

----------------------- New Test -----------------------
Paessler SNMP Tester 3.2
Device: 10.9.1.1

Scanning Standard Interfaces...
3/15/2012 12:12:25 PM Start Scan
3/15/2012 12:12:25 PM SNMP Version: V1
3/15/2012 12:12:25 PM Delay: 0
3/15/2012 12:12:25 PM Force32: No
3/15/2012 12:12:25 PM Single Get: No
3/15/2012 12:12:25 PM GET: 1.3.6.1.2.1.2.1.0
3/15/2012 12:12:25 PM Try 1
3/15/2012 12:12:25 PM Start 3261440
3/15/2012 12:12:25 PM Done 3261440 Status=1
3/15/2012 12:12:25 PM Try 2
3/15/2012 12:12:25 PM Start 3261440
3/15/2012 12:12:25 PM Done 3261440 Status=1
3/15/2012 12:12:25 PM Try 3
3/15/2012 12:12:25 PM Start 3261440
3/15/2012 12:12:25 PM Done 3261440 Status=1
3/15/2012 12:12:25 PM Try 4
3/15/2012 12:12:25 PM Start 3261440
3/15/2012 12:12:25 PM Done 3261440 Status=1
3/15/2012 12:12:25 PM Try 5
3/15/2012 12:12:25 PM Start 3261440
3/15/2012 12:12:25 PM Done 3261440 Status=1
Result: #N-12

Found standard interfaces:
No standard interfaces found

About my hardware and software installation: We using a pair Cisco Catalyst 3750G 48-port switches combined in stack with original stack cable and two Cisco catalyst 3750G 24-port switches in the stack too. Stacks configured as network core.

First pair (48-port) switches is located in the Data Center and the second pair (24-port) located in the Office. We have beetween these "Network cores" a stable 1 gigabit L2 link.

PRTG monitoring software installed on virtual machine under Microsoft Hyper-V and running Windows Server 2008 R2 Standard 64-bit. It have 4 processor cores Xeon E7340 and 4 gigabytes RAM. PRTG server and first probe are installed on the same machine, second probe installed on the different server but not used by any device or sensor. All sensors depended to first probe with server installation on the same virtual machine.

PRTG and its components are: Server Service: V9.2.0.2230 Probe Service: V9.2.0.2230 Server Administration Tool: V9.2.0.2230 Probe Administration Tool: V9.2.0.2230 Import Tool: V9.2.0.2230

What I need to do to resolve this issue?

Thanks in advance and kind regards, Alexey Gosha

cisco-3750 ping- snmp wmi-service