During the last post I mentioned that we needed rsh in order to fallback as the ssh apparently didn't work.
Now when the database should be build with the dbca an error was thrown, stating that the user oracle had no user equivalence on its own node.
Searching Metalink revealed that the dbca was looking for ssh in /usr/local/bin/ssh .
However, ssh on AIX is located at /usr/bin/ssh .
Created a softlink solved this problem.
This could also be the explanation for the problems I had earlier. When setting up the user equivalance with ssh you use the commandline. As /usr/bin is in the path before /usr/local/bin from the commandline the ssh works just fine. When this is hardcoded in the Universal Insstaller or the dbca you could get these problems.
BTW: it seems that this is solved in 10.2.x