WebDav vs Microsoft Office

Expected behaviour
Ms office ask usename and password

Client configuration
WebDav Win 7/10
Server configuration
Operating system:Debian 8.7
Web server:Apache2
PHP version:5.6
ownCloud version (see ownCloud admin page) 9.1.3:
Updated from an older ownCloud or fresh install fresh:
Trusr sert by letsEncrypt

I had this problem with a previous webdav-setup and had the impression that office is opening it's own webdav connection. Can you see the client user agent for these calls in your webserver logfiles?