How can a Cassandra node see another node as down?
Running nodetool enablegossip on the host that appeared down to other nodes fixed it for me and for now. However, it appeared as down to all other nodes I checked. Running in a non-cloud environment.

I was curious what my other nodes said and I found one that had the same issue (like your 10.0.9.8, showing 10.0.10.8 as down). Only running nodetool enablegossip on 10.8 didn't help. But running disablegossip first and then enablegossip again did!