Last week i got following error-stack when trying to open the Enterpris-Manager:

503 Service Unavailable
Servlet error: Service is not initialized correctly. Verify that the repository connection information provided is correct.
conn.ConnectionService verifyRepositoryEx.818 - Invalid Connection Pool. ERROR = ORA-00376

When digging deeper into the error-logs i found the problem that a datafile was not online.

ORA-01110: Datendatei 5: 'C:\ORACLEHOMES\ORADATA\EMREP\MGMT.DBF'
ORA-06512: in "SYSMAN.MGMT_USER", Zeile 10174
ORA-06512: in "SYSMAN.SETEMUSERCONTEXT", Zeile 10
ORA-06512: in Zeile 1

I was asking our admin, and he tried a new file-backup that locked the datafile while oracle was trying to write to it.
So it went in "recover" mode.
Solution for this issue:

Select the files in recover-mode    : select file#,name,status,enabled from V$datafile;
recover them                                  : recover datafile 'XXX';
set them online                               : alter database datafile 'XXX' online;