Jumping in here... I think this may be similar to what I've heard someone else having a problem with (I'll try to get them to comment).. They upgraded to a new code version and enabled FIPS mode which forced only certain types of crypto protocols for SSH which from my understanding NCM was not capable of using or not set to use. That explains why putty from the Solarwinds server will work, but NCM itself can't get in (because it doesn't have the required crypto level). I suggested they open a support case but never heard back from them.
Before that, does your device by any chance happen to be running in a FIPS mode or running some kind of strict crypto/SSH/key requirements?