Sample Error Message:

ERROR main 2019-09-18 09:47:39,700 Expression.java:212 - Can't cast value for myColumn to size accepted by myValidator, value size is 1024.

What does this ERROR message mean?

This error means a value was included in a query or insert which was not of the type expected, and/or could not be cast to the data type of the SASI index.

Why does this error occur?

This error shouldn't be possible, as the validation is done previously at the CQL layer. If it happens, that means a check was failed along the way on some invalid data. It may be possible for this to happen if you load data from files with different schema versions, for example. Note, this error is associated with SASI indexes specifically

How do you fix this ERROR?

First verify the data you're querying, or loading is correct and expects the current schema. If you get this error otherwise, it may be a bug and further research will be needed.