After searching, testing and reading it is now getting clearer.
The problem I encountered is NOT an NEXTCLOUD issue, but has to do with the confusing (at least for me) PHP handling my Synology Diskstation does (Nextcloud is running on it).
The PHP-CLI uses a different version php.ini file (located in /urs/local/etc/php70/) than the webstation does (located in /volume1/@appstore/WebStation/misc/). So configuring the PHP version for the Webstation does only affect the php.ini from the WebStation which is used to run Nextcloud in the browser. This means that the webupdater could behave different from the occ-script launched via terminal, even if both run PHP 7.0.
To get things work I first need to unterstand this.
The PHP configuring via the GUI of DS > WebStation only affects the PHP-version that is used by the web updater.
The configuration of the PHP-version used by the CLI, to run occ commands needs to be set in the php.ini in the /urs/local/etc/php70/ folder. Most missing modules can easily be enabled by just adding ‘extension = pdo_mysql.so’, … to the php.ini-file (e.g. using midnight commander as root (sudo -i)).
I hope this might help You, when You encounter errors like the following:
Uncaught Doctrine\DBAL\DBALException: Failed to connect to the database: An exception occured in driver: could not find driver in /volume1/web/nextcloud/lib/private/DB/Connection.php:64
At least on Synology, which a few of us run Nextcloud on, migth help to unterstand whats wrong, and where / how things could be solved.
Happy New Year and happy nextclouding.