Please help us by providing the following info.
Steps to reproduce
1. Install client (from repository)
2. Try to log in
You get logged in
Get error about username / password
With the SAME username and password I can log in on the iOS and Android app and on the web-page.
Client operating system: Raspbian (Debian flavour for Raspberry Pi)
Operating system: Raspbian
ownCloud version (see ownCloud admin page): 9.1
Updated from an older ownCloud or fresh install: Fresh