Hey @Ingo_Richtsmeier,
that was exactly the issue. After I rewrote those context statements to retrieve the services via injection, everything worked perfectly. When I was reading the exception message, I somehow completely forgot to check the line 32 in my CandidateListener class 
Now everything seems to be working fine, unfortunately I am not able to find that much documentation for the spring boot starter.
Viele Grüße zurück! Was für ein Zufall.