Hi Venkat and thanks for the answer!
No, the database wasn't installed at all. I have installed it manually after sapinst.
Then the problem with xuser not found came. Yesterday I did some additional research
on this webpage and I found the 4 commands to recreate the users too. But most probably
I don't have a database at all, so I'm really not sure how to create the DB for SAP and
there is TOTALLY NO DOCUMENTATION on that !!
And in the end, it remains a total mystery, why sapinst didn't do nothing about the DB.
Although it asked about the RDBMS DVD and it accepted the location without any errors.
I have tried to use newer sapinst versions, but I couldn't even start an installation with them.
I've also checked the SAP note about the Software Provisioning Manager, but it seems to
support only versions 7.0 and above.
PLEASE HEEEEELP :-)