I've seen Oracle VARCHAR versus NCLOB errors, but usually on synchronize database.
The trick there is to alter the type before the startup of Mendix/synchronize database. Here is an example how to change the type from VARCHAR to NCLOB. If the reverse is needed you need to find out the size in the modeler to do a VARCHAR2(200) for example.
ALTER TABLE "oracle table name" ADD ("temp column name" nclob);
UPDATE "oracle table name" SET "temp column name"="error column name";
COMMIT;
ALTER TABLE "oracle table name" DROP ("error column name");
ALTER TABLE "oracle table name" RENAME COLUMN "temp column name" TO "error column name";