[EN] ORA-01111: name for data file n is unknown - rename to correct file

PROBLEM:

On one of our physical standby database, we noticed that media recovery was stooped.
When we try to start it again we got the following errors in altertlog;

ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE DISCONNECT FROM SESSION
Attempt to start background Managed Standby Recovery process (PRODSTB)
Fri Aug 17 09:11:05 2015
MRP0 started with pid=39, OS id=8061128
MRP0: Background Managed Standby Recovery process started (PRODSTB)
started logmerger process
Fri Aug 17 09:11:10 2015
Managed Standby Recovery starting Real Time Apply
MRP0: Background Media Recovery terminated with error 1111
Errors in file /u01/app/oracle/diag/rdbms/prodstb/PRODSTB/trace/PRODSTB_pr00_7985076.trc:
ORA-01111: name for data file 24 is unknown - rename to correct file
ORA-01110: data file 24: '/u01/app/oracle/product/11.2.0.4/db/dbs/UNNAMED00024'
ORA-01157: cannot identify/lock data file 24 - see DBWR trace file
ORA-01111: name for data file 24 is unknown - rename to correct file
ORA-01110: data file 24: '/u01/app/oracle/product/11.2.0.4/db/dbs/UNNAMED00024'
Managed Standby Recovery not using Real Time Apply
Recovery Slave PR00 previously exited with exception 1111
MRP0: Background Media Recovery process shutdown (PRODSTB)
Completed: ALTER DATABASE RECOVER MANAGED STANDBY DATABASE USING CURRENT LOGFILE DISCONNECT FROM SESSION