What does this error mean?

As the error states, an unknown exception was thrown when attempting to write to a Materialized View table.

Why does this error occur?

If a write requires an update to the Materialized View, Cassandra tries to write to the view and push out the update to replicas. If the process throws an error of some type, it is caught and the error message is displayed along with information about the partition and the throwable error.

How do you fix this error?

Because this is catching any throwable error, you will need to look at the full exception message and stack trace to determine the underlying cause. If you need help with this step, please reach out to DataStax Support.