Blank screen after update nc 11 to 12

Nextcloud version : 11 upgraded to 12
Operating system and version _(raspbian jessie lite):
Apache or nginx version (eg, Apache 2.4.25): nginx
PHP version (eg, 5.6): php5.5
Is this the first time you’ve seen this error?: yes

The issue you are facing:
Felt like I had to upgrade from 11 to 12…

Did the web-based upgrade. Continued with maintenance-mode on via putty and completed upgrade. Since that only a blank screen appears, when I try to log in with a browser oder a client app.

restarted nginx and php services.

The output in /var/log/nginx/error.log:

2017/06/26 18:46:35 [error] 3560#0: *158 FastCGI sent in stderr: "PHP message: PHP Fatal error: Class ‘OCA\DAV\Connector\Sabre\ExceptionLoggerPlugin’ not found in /var/www/html/nextcloud/$

Actually I just don´t know where to start fixing it…

Thank you!

PHP 5.6+ is required, PHP 7+ recommended:

If the issue persists: Most often I see blank screens in combination with wrong memory caching configurations. Also with NC 12 it became stricter, e.g. APC is not usable anymore in config.php, just APCu instead.
Could you provide your config.php in case?