ORA-01640: cannot make tablespace read only with active transactions
Cause: Attempting to make a tablespace read only while there are active transactions in the database. All transactions must commit or rollback to insure that there is no undo for a tablespace before it can be made read only. This includes in doubt distributed transactions.
Action: Prevent any more transactions from being started. Putting the database in restricted mode usually helps. If there are any in doubt transactions they must also be resolved.