Well, problem <partly> solved !
I did the reinstall of the X-server, as mentioned here, but to no avail.
(mind you, during install there was a constant warning that ".usr/lib/Libtdb.so.1 is NOT a symlink")
I looked into some X error logs and found no flaws whatsoever.
The only error that ever occurred was during boot-up concerning NFS.
NFS started fine, but connecting to the shares showed an error, but again did not interfere with the boot process.
So for a try I moved "nfs" and "nfsmount" out of /etc/init.d and rebooted.
And now I'm posting this from Firefox AND Sabayon.
Which makes Sabayon not very usefull to me at the moment since all my data is on the NFS-shares
Let's see if I can get NFS working, but all about that in another thread.