1. The lock request or an implicit lock conflicts with the locks of another user. The maximum waiting time for being granted the lock has elapsed (special database parameter REQUEST_TIMEOUT).
2. The database instance could not be stopped in a certain time interval, because transactions with SQL statements for data modification are still open.
3. In some cases, the error message contains a more detailed description of the error. The lock request can be reissued. To avoid possible deadlock situations, it is advisable to roll back the transaction using a ROLLBACK statement.
4. Do without the shutdown, or force a shutdown of the database instance using the QUICK option.