The problem is solved. 
- upgrade the Ubuntu server to 18.04
- Install php7.2 (php7.2-bz2, php7.2-imagick…)
- completely uninstall php7.0
- disable onlyoffice (sudo -u www-data php occ app:disable onlyoffice)
With this Owncloud is again accessible and usable.
I will take care of onlyoffice later.