The error message is caused by a bug at compression of varchar-columns prior to the version 4.2.5. Under some specific circumstances, the same string will be transformed internally to two different compressed presentations during an insert operation.
Reading the value causes normally no issues as it will be decompressed into a consistent presentation.
If there's however a simple filter like '<column> = <constant>', EXASolution works directly with compressed values to speed up the query execution.
In this case, only the rows matching to one of the compressed representations will be found.
The data inserted by the version 4.2.5 and above won't be affected by the bug. The version 4.2.5 also detects erroneous values and interrupts the execution displaying the error message above.