-25323: Data type of null value representation for column '<column_name>' is incompatible with the column data type 

Explanation:

Error executing the Loader command for exporting data. For individual columns you can specify a NULL value representation in the command. This means that if a NULL value is exported, the constant value entered in the command is entered into the data stream. The data type of the constant value you specified and the external data type of the column to be exported must be compatible. If you do not specify a data type in the command, the default data type CHAR is used.

User Response:

Correct the command and then re-execute it.