Cassandra: occasional permission errors
Is the system_auth keyspace RF the same as the amount of nodes? Did you try to run a repair on the system_auth keyspace already? If not do so.
For me it sounds like a consistency issue.
I use cqlengine with django. In some occasions Cassandra throws an error indicating that user has no permissions do to something. Sometimes this is select, sometimes this is update or sometimes it is something else. I have no code to share, because there is no specific line that does this. I am very sure that user has all the permissions, and sometimes it works. So if user did not have the permissions it should always throw no permission error. 
So what might be the reasons behind this and how to find the problem?