write access is not allowed if RDB$SYSTEM is read-only

  Explanation:  BATCH UPDATE and EXCLUSIVE UPDATE access do not
  update the snapshot files.  The fact that snapshots are not
  being maintained is recorded in the RDB$SYSTEM area.  Hence,
  RDB$SYSTEM may not be READ_ONLY.

  User Action:  Use another UPDATE access mode, or change
  RDB$SYSTEM to be READ WRITE.