[Answered ]-Cassandra: occasional permission errors

If you have authentication enabled, make sure you set appropriate RF for keyspace system_auth (should be equal to number of nodes).

Secondly, make sure the user you have created has following permissions on all keyspaces. {'ALTER', 'CREATE', 'DROP', 'MODIFY', 'SELECT'}. If you have the user as a superuser make sure you add 'AUTHORIZE' as a permission along with the ones listed above for that user.

Thirdly, you can set off a read-repair job for all the data in system_auth keyspace by running CONSISTENCY ALL;
SELECT * from system_auth.users ;
SELECT * from system_auth.permissions ;
SELECT * from system_auth.credentials ;

Hope this will resolve the issue !

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.