Here is my situation:

 

I've had snmp working fine on my switches until recently.  The change relates to the switch's management IP address.  My snmp credentials fail when monitoring the switch using the new IP:

 

old IP

10.99.1.10

255.255.248.0

 

new IP

10.1.99.10

255.255.255.128

 

I can work around the issue by deleting and then re-adding the switch.  My best guess is that the problem is in the different subnet masks.  I can tell Opmanager to monitor the switch using the new address (and I can even ping the switch), but I do not know where to change the mask.  I could be totally off on this one, but that is where I am at the moment.  I will post updates as I research this issue further.