This could indicate a device bug, or some issue with ACLs. Pick one such failing device. What type of device is it? What version of code is it running? How did you verify the credentials were correct?
To troubleshoot further, perform a Device Credential Verification job to this device while running a packet capture filtering on all udp/161 and tcp/22 traffic to this device. The problems with the RW community string should be clearly visible in the trace. There may also be some SSH information that can be ascertained as well.
Additionally, if you enable Device Credential Verification Jobs debugging under RME > Admin > System Preferences > Application Loglevel Settings, the job log may have additional information as to why the operation failed.