To use adb sideload, i installed ‘Minimal ADB and Fastboot’ (http://forum.xda-developers.com/showthread.php?t=2317790) on my Windows 10 computer, which worked fine.
Now i could transfer files to my FP2, but i think i misunderstood the mentioned solution
https://forum.fairphone.com/t/stuck-optimizing-apps-after-updating-on-fp-open-os-16-07-1/21146/2?u=chrisr
I thought that i should reinstall the Fairphone Update via TWRP (which did not solve the problem) but instead of that i had to reinstall the Gapps via TWRP.
Now everything works again.