Hello Jan,
I see you have a CSS ticket open with SAP.
I tried to reproduce the error (-3004) you mentioned in our environment and I was able to reproduce it when I try to restore the "Incremental Backup" using a medium defnition created for "Data Backup"
It looks like from the output you attached
COM D:\Backup\VOL\DB2_INC FILE DATA 0 8 YES NO
20130322121825 20130322121825 NONE
"COM" is a medium definition for the "Complete Data" Backup - but the Device/File for this medium is pointed to the incremental backup "DB2_INC" you need to change the medium definition to the File location where you have the complete data backup. You can do this using the DATABASE STUDIO during the restore process - select the medium "COM" and click on EDIT.
This should fix the problem. If not, please open a WTS connection to the system and update the CSS message - I will take a look into it.
Regards,
Yashwanth