$ dig nedgros.de any
;; QUESTION SECTION:
;nedgros.de. IN ANY
;; ANSWER SECTION:
nedgros.de. 86301 IN CNAME nedgros.herokuapp.com.
Short answer: you cannot have a CNAME at the "top" of the zone, a CNAME is an alias or reference, it cannot coexist with any other data. Specifically, it cannot coexist with a NS and SOA records which are rather important for the correct operation of your domain. RFC 1912 Section 2.4.
You have not properly followed the Heroku instructions. See https://devcenter.heroku.com/articles/custom-domains , if this setup is what you want read the part on "Apex domains".