Why can’t my custom domain on Heroku be resolved?

I configured a custom domain for a small site on Heroku on Friday. It worked then, but now, it's broken. The host is nedgros.de. nedgros.herokuapp.com has address 23.21.239.236.

Is does not resolve to the right application. How can I configure the DNS to follow the Dyno dynamics?

$ 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".