After update to debian jessie: 404 on all drupal sub-pages

[Markus Grunwald <markus@the-grue.de>]

Hello,

when updating from wheezy to jessie, I had some trouble. I followed the release
notes, but since I'm not a php developer, I didn't understand everything. Now,
there's trouble:

The release notes mention something about php 5.6, but as a simple user of
drupal I don't understand the meaning:
https://www.debian.org/releases/stable/amd64/release-notes/ch-informatio...

First, I thought all is fine, as these drupal pages of mine worked ok:

http://www.goldschmiede-grunwald.de/
http://www.maennerchor-kirchseeon.de/

Today came the panic, as I recognized that every drupal-page below / gives me a
404, for example:

http://www.goldschmiede-grunwald.de/impressum
http://www.maennerchor-kirchseeon.de/impressum

Pages that are not drupal-related work ok, too:
http://www.goldschmiede-grunwald.de/gna.php
http://www.maennerchor-kirchseeon.de/download/

Owncloud, another php software works perfectly on the same server.

Static pages from the same server are ok, too:
https://www.the-grue.de/mail_und_co/komischer_anhang.html

I put the log output of my apache to debug level, but everything I see is just
the 404 message:

178.26.130.51 - - [27/Sep/2015:14:51:49 +0200] "GET /impressum HTTP/1.1" 404
518 "http://www.goldschmiede-grunwald.de/"; "Mozilla/5.0 (X11; Linux x86_64)
AppleWebKit/537.36 (KHTML, like Gecko) Chrome/44.0.2403.107 Safari/537.36"

I already tried for a few hours, but now I've completely run out of ideas.
Could you please help me?
-- 
Markus Grunwald
https://www.the-grue.de

Fragen zur Mail? https://www.the-grue.de/mail_und_co
https://www.the-grue.de/~markus/markus_grunwald.gpg