Test monitor fails while snmp request using MibBrowser works

I'm evaluating OpManager before buying it, and I have some performance monitors problems for Linux servers.
I have the "Snapshot view displays "No Data" for CPU, Memory or Disk utilization" problem, I've checked the "Troubleshooting Snapshots and Graphs".
When I use the Mib Browser, set up host and snmp community, and request ".1.3.6.1.4.1.2021.11.11.0" using "UCD-SNMP-MIB.mib", it works fine and I get the answer.
If I go on my server in OpManager, to the Performances Monitors, edit the CPU utilization (the one using UCD mib), then click on "Test monitor", it fails with this message: "The Monitor did not respond to the test request". In SNMP parameters, timeout is set to 20 seconds, but the answer needs less than 3 seconds using MibBrowser.
How could I fix this problem?

I'm using OpManager 7 pro trial, on a Windows 2003 R2 standard server, and the linux server I'm trying to monitor is running RedHat Enterprise Linux ES 4 update 4.
The UCD mib has been taken from /usr/share/snmp/mibs on the linux server, to be sure to have the right mib

Thank you