Webinterface defect after update or fresh install NC12

Nextcloud version (eg, 10.0.2): 12.0.0
Operating system and version (eg, Ubuntu 16.04):Raspbian Stretch / Raspbian Jessie
Apache or nginx version (eg, Apache 2.4.25): Apache 2.4.25-3
PHP version (eg, 5.6): 7.0
Is this the first time you’ve seen this error?: yes

Can you reliably replicate it? (If so, please outline steps): yes
first I was running an update from NC11.0.3 with command line updater on fresh dist-upgraded raspbian jessie.
When I saw the webinterface not working, I started from ground and set up a new Raspbian stretch and installed a fresh NC12 following this guide with the same result.

The issue you are facing: The Webinterface is only text-based, no icons, unusable.