Oracle Error Message
ORA-39300: Cannot alter materialized view log partitioning type or partitioning columns.
Reason for the Error
DBMS_METADATA_DIFF was comparing two materialized view logs with incompatible partitioning: (1) one is partitioned and one is not (2) both are partitioned but in different ways (locally vs. globally, hash vs. range) (3) both are partitioned but on different columns. There is no SQL ALTER statement to change the way a materialized view log is partitioned.
Solution
The difference cannot be eliminated with an SQL ALTER statement.