Allright, the problem was that smack-java7 had no OSGi declarative service for its initializer, which cause the Base64 decoder not to be configured. This should be fixed with Add smack-java7 declarative service · 991d39a · Flowdalic/Smack · GitHub
I've also just uploaded a new alpha2-SNAPSHOT containing the changes.
@Jens: As always, could you try and report back? It's my understanding that the initialization order problematic is still not solved, correct?
How did you declare the Smack dependency? It should be sufficient to just depend on smack-java7, as all other required artifacts are then pulled in automatically.
Best
Florian