An error occurred while processing the online backup.

Simulation
Alter tablespace begin backup;
Shutdown abort

Procedure:
1. SQL> startup force Mount;
2. determine all data files that are online
SQL> SELECT FILE # from V $ backup where status = 'active ';
File #
-----------
4
3. End the online backup status (three methods)
A) multiple data files are backed up online: Alter Database End backup;
B). An online backup of a data file: Alter database datafile... End backup;
C). Recover datafile 4;
4. Open the database
When the database is opened, the background SMON will automatically execute routine recovery.
SQL> alter database open;