WebDAV: how to debug the config?

This forum collects a huge count of issues with webdav. But there is not summary what has to checked if webdav does not play-


Nextcloud version: 16.0.3
Operating system and version (eg, Ubuntu 17.04): openSUSE LEAP 15.1
Apache or nginx version (eg, Apache 2.4.25): apache 2.4.33
PHP version (eg, 7.1): 7.2.5

Where are the points that should be checked if the webdav does not connect? Which config files should contain what lines? Which log can give a closer look?