There should have been no problem with the logs. Or the would have been deleted by restarting ISP config server.
I have removed httpd and dependencies, rebooted server, installed httpd etc. again and rebooted the server, still httpd did not want to start. So I removed httpd again.
I then compiled apache by hand, instead of using yum. It was installed into /usr/lib/httpd. Now I installed httpd via yum again. A miracle must have hapened, I could start httpd again. It might be caused by compiling apache manually... But now two apache's are configured, while yum install httpd overrode some of the manual install I think. To restore the old situation, I would have to decompile apache, then yum install httpd again. I wonder if it still would have problems, but I'm not going to investigate such.
There was enough disk space, about 500 M available. Also the syntaxis of the configs was OK.
Thanks for all help, I decided to start a new server with chooted lighttpd this time :-) (it just looks like I'm searching for trouble ;-))
In any case I now use webmin instead of ISP Config, webmin is more convenient imho.
P.S. @ forum admin, you really should look at the problem when being logged out while writing a reply, I have to log in again, but then it says invalid thread specified. Fortunately FireFox had the text saved.