Your example is correct and if you solved the whole puzzle right it should gives you the 'correct' error message which is different with the 'wrong' error message
I've found my issue.
I wrote a python script for the decoding and for the shift of the numbers there was an issue if it's below 0 :D, as I wrote this with +5 and not -5 in mind 
try -5 instead of +5
I've already tried that, unfortunately also the wrong error message.
This is my example with -5:
Input : 8Fg6k
Output: 3Ab1f
Ich bekomme leider die falsche Fehlermeldung, ich vermute ich mache irgendwas beim entschlüsseln falsch ...
Kann mir jemand sagen ob die Verschiebung für folgendes Beispiel passt?
Input: 8Fg6k
Output: 3Kl1p
