@myii Thank You for the hint! The phone was "clean" (resetted to factory settings) when I flashed it so there was nothing but the android-system on it - but I think I´ll rather reinstall and choose wipe-option this time...

@joe The description next to the mouse claims it's for Windows 8.

@neopar Thank you for that link. Yet I hope an UI approach for CardDAV will be available sometime, the CalDAv works fine already. I have no interest in putting up further server logistics and traffic while my email-provider (posteo) does all that already in a safe manner. I found: https://docs.ubports.com/de/latest/userguide/advanceduse/dav.html and https://forums.ubports.com/post/4916 ...but I have no experience in this sort of scripting or linux in general if you will (windows user). Nevertheless in order to try that out now I first have to figure out why the phone is on the one side recongised by windows (attached to usb and no errors in the device manager but still e.g. no diskspace available) but on the other side I cannot "talk" to it via adb ("adb devices" gives back an empty list whilst phone-debugging is enabled). I´ll have a break now and come back to it later...