Migrate drupal from php5 to php7

The Website http://mainzer-kammerspiele.de is using 5.5.38 and sould be upgraded to use php 7.0. A copy of the site is running at http://mainzer-kammerspiele.cross-solution.de

At least the login and pagination does not work.

Example pagination: http://mainzer-kammerspiele.cross-solution.de/tags/schauspiel?page=2

Example login: http://mainzer-kammerspiele.cross-solution.de/user/login

A login can be created on the shell via:

root@mainzer-kammerspiele:/var/www/mainzer-kammerspiele.de/docroot/sites/default# drush help ucrt
Create a user account with the specified name.

Examples: drush user-create newuser Create a new user account with the name newuser, the email address person@example.com, and the password letmein --mail="person@example.com"
--password="letmein"

Arguments: name The name of the account to add

Options: --mail The email address for the new account --password The password for the new account

Aliases: ucrt