cannot mount database in exclusive mode

 Hello,
On Oracle DB release 9.2.0.7.0   we had some problems after a shutdown and restart.


Connected to an idle instance.

SQL> ORACLE instance started.
...
ORA-01102: cannot mount database in EXCLUSIVE mode

Then when reconnecting:

SQL> grant select on v_$sysstat to SIGSTCLU
*
ERROR at line 1:
ORA-01109: database not open

.....
SELECT 'DB_ROLE:'|| DATABASE_ROLE FROM v$database
*
ERROR at line 1:
ORA-01507: database not mounted


DB_STATUS:STARTED
Role retrieval failed.

After another shutdown and restart all is well. Could you give me some tips what could have gone wrong?
Are there any logfiles I could get more information what happened?