for all those who are having the same problem, try the following:
After receiving the message “An unexpected error occured….” close the “Upload new APK to production”-window. Now you should be back to the APK screen. After refreshing by pressing F5 you should see the new and the previous APKs and another errormessage. Disable the old AKP manually and on the upper left should finally appear the “Publish APK”-button.
Thank you Alex, for opening my eyes 😉