Clients (Android & Win) can't connect

Hi there!
I've done hours of research, but can't figure out, how to get the client(s) running (Win&Android).
Server at 1&1 SQL/appache. I can login via the webInterface but not with any client. Figured out it might be a WebDAV problem, but I honestly have no clue how to solve it. Hope any of you guys has an idea ...

Actual behaviour

Server is accepted by client but doesn't accept password/username

Expected behaviour

Connection should be established.

Environment data

Android version: 5.0.2

Device model: MotoE2

ownCloud app version: 2.3.0

ownCloud server version: 10.0

Yes, such issues are mostly caused by some mis-configurations on server side. Unfortunately there is no single solution for this update. Because of this an quite large FAQ exists at [1] where you probably need to work your way through to solve this issue.