I finally upgraded to SIM 7.2.2 (latest version I can run due to IRSA) and shortly after, I noticed that discoveries and identify tasks fail with systems running SNMP. I also tried using the sytem types tool to pull OIDs from systems and it fails as well.
I stumbled across a blog where someone found that a new parameter existed in the globalsettings file - "JavaSnmpStack", and setting it to 'false' fixed the issue for me - for about 15 minutes! Without doing anything else, the same issue came back.
It's very strange.. yes the clients have valid SNMP community names and yes the SIM credentials are correct - i have thousands of devices so they all could not be wrong.
Thanks