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.