The following Knowledge base article explains why you are getting this, how to temporarily fix it and how to permanently fix it:
Chris Foley - SolarWinds - Support Specialist
Support: 866.530.8040 | Fax: 512.857.0125
network management simplified | solarwinds.com
I had this problem with some virtual AIX servers after dynamically changing the number of CPUs. The SNMP agent on AIX wasn't smart enough to detect this change, but after restarting the SNMP agent the problem went away.
Maybe restarting the SNMP service/agent/daemon on the device that you are monitoring will fix your problem?