I found my mistake: The password I had for my users was too short! I didn't check it before because I didn't see anything about password requirements in the documentation of lotus connections. I had a password with just four letters and obviously that wasn't enough. It was just by chance that I tried an user with a longer password.
And if anyone might be having problems with the oracle database connections: I got ok testing results for the database connections but something with the blogs and homepage database still wasn't going well. I tried other users for the JAAS-aliases for both of them and all was good.
The database scripts generate two different users for each component in oracle.
At first I took the usernames with -user attached to the name ("filesuser" etc.) but for blogs and homepage I just needed the users "blogs" and "homepage". It's a little bit confusing, I found the correct users in the Lotus Connections 2.5 Installation Guide (http://www-05.ibm.com/e-business/linkweb/publications/servlet/pbi.wss?CTY=US&FNC=SRX&PBL=GC14-7258-00
) on page 134.
Thanks again for your help,