Hi Eric!
Thank you VERY much for your reply!

Here's the answer to your questions:
1) Yes, we generated a password file.
2) Yes, we performed a relink. After the relink we went to the Oracle Backup
Manager and this showed that a new channel was created. Before the relink we
could only select a channel type DISK, after the relink we could select both
channel types DISK and TAPE.
3) Yes, Oracle 8.0.5. is supported (source:
http://www.storage.ibm.com/software/adsm/adagent.htm#OracleSolaris)

Now your suggestions:
1) sbtio.log shows: (16222) OBK-sbt: sbtinit: initializing SBT API
So this looks ok.
2) No dsierror.log exists
3) No entries are made in the server activity log.

Let me see if I understand the backup agent correctly.
RMAN initiates a backup to a channel.
When you use RMAN to backup a database with channel type DISK a sequential
disk file is created.
During the ADSM Agent installation you create (during the linking step) a
channel (type TAPE) which is used for ADSM backups. When one initiates
backup through RMAN while using the ADSM created channel type, the backup
should go directly to ADSM.
When we initiate a backup to the channel type TAPE, RMAN still creates a
disk file.
So it looks like something is going wrong here.
Any ideas on this problem?
Any reply is GREATLY appreciated!
Kindest regards,
Eric van Loon