Shared Links broken after upgrade

Server
1

Hello, I have upgraded our Owncloud as described.

After update all "Share links" are broken. When I try to access
a link I get a password login. I also noticed that the links
have changed

from http://cloud.domain.de/index.php/s/1de91c5bf570594fa920ef6b89866632
to http://cloud.domain.de/public.php?service=files&t=1de91c5bf570594fa920ef6b89866632

Is it possible to make the old links work again? Can someone advice?

Steps to reproduce
1. upgrade from 7.0.1.1 to 9.1.4.2

Expected behaviour

Actual behaviour
All user Share links are broken

Server configuration
Operating system: debian 8
Web server: apache 2.4.10
Database: mariadb 10.0.29
PHP version: 5.6.30
ownCloud version (see ownCloud admin page): 9.1.4.2
Updated from an older ownCloud or fresh install: 7.0.1.1
Special configuration (external storage, external authentication, reverse proxy, server-side-encryption):

ownCloud log (data/owncloud.log)

Please paste possible errors in the following code block, see https://central.owncloud.org/t/how-to-find-webserver-or-oc-logfile-enable-php-logfile/808 for more info

Integrity status for oC9+

No errors have been found.
2

Can you show us the content of owncloud.log during clicking on the link, please?

3

That's the next issue I can't fix. The owncloud.log is beeing generated
but it remains empty regardless what level I chose.