Ok, while I wrote this, I also resolved it, so I write it anyway, maybe it helps someone:
[The short version: activation seems to use more than 32MB of memory for PHP for me, which was over the default of what PHP is allowed to use. Change it, and it works. Long version below:]
I already tried to disable all plugins and disable the theme, did not work.
Jetpack 1.9.2 <- or better: I want to use that
Smart Youtube PRO 4.1.7
Grey Opaque 1.8.5
as I said, I already changed the theme (I did not remove it, just switched to the other) and disabled all modules. As soon as I try to aktivate jetback in the plugins area, the screen goes blank with that url:
Now after looking into my error.log (tail -f /var/log/apache2/error.log) file I found the following:
"PHP Fatal error: Allowed memory size of 33554432 bytes exhausted (tried to allocate 32 bytes) in /srv/www/li305-99.members.linode.com/public_html/wordpress/wp-includes/pomo/translations.php on line 256, referer: http://blog.jsimon.at/wp-admin/update.php?action=install-plugin&plugin=jetpack&_wpnonce=dfa2f834b6"
So, older servers with ubuntu have a default of 16MB, newer of 32MB, and jetback seems to overrun that now.
Usually setting the memory_limit variable to something higher should work:
sudo vi /etc/php5/apache2/php.ini
Hope that helped someone ;)