Ok folks, my phone is now in quite good shape again.
Android 6 is running, all my apps are back (not their settings …) and i’ve got my contacts and SMS back.
Here’s how i did it:
First, i followed the instructions in the post i’ve mentioned above.
So, i installed Version 17.04.0 (phone still in a boot-cycle) and immideatly after that Version 17.06.4.
After that, Android was running stable.
Then, i restored my contacts, SMS and call-log from the TWRP-Backup (which was creatad after the failed update, so there’s hope if your in this situation)
There’s an incredibly good post here:
Not everything works as explained though, so maybe i’ll give some hints here:
1) In the .tar.gz-file, the contacts2.db is located here:
while the mmssms.db is located here:
Also, you can use the search within winrar (or whatever you’re using).
2) The Call Log Backup & Restore App is unessescary, SMS Backup & Restore will do the trick (furthermore, the call log app isn’t available in all countrys vie the play store …)
3) There’s no need for extra software to convert the .csv (for the contacts). The mentioned Excel-sheet comes with two macros. One to produce a .csv-file and one to convert said file into a .vcf-file
So i hope, this will help you, if your phone doesn’t work after the update.
@Fairphone: If the problem is, that i’ve not installed the “first” version of Android 6 before the newest update, shouldn’t there be a prompt before the update. I’m not a developer, so maybe that’s not the best idea, but maybe …
Thanks again to everyone who helped me.