Alain Raap

Did you try dbtype "mariadb"? Can you reach the database with mysql command as root?

Could you share some information of your database logfile? Usually it gives some more information about the error you get.   

Jae Ann you're welcome and hth! Howard you were right, phpinfo is a better way to check the php version on the server. Thanks for the tip!

Another problem could be that the old php version is still active while php 7 installed. Check this with the (/usr/local/bin/)php -v command. I removed the complete php 5.6 version packages from my Linux system and then my latest version was 7.1.

Are you using a caching server like Memcache? You could also try to purge the caches in Moodle (inside admin/cli directory or via the Admin console), sometimes this can help to solve errors.