Thanks for you reply Jan.
As I wrote before it is not possible to enabled DEBUG mode in alpha6 (on Android).
It is possible, but since alpha6 Smack uses JUL for logging. And per default JUL does not output log messages of level debug. So you need to either change the JUL settings to output also log messages of level FINE, or you could disable the JULDebugger with
SmackConfiguration.addDisabledSmackClass("org.jivesoftware.smack.debugger.JulDeb ugger")
which should cause the AndroidDebugger to get selected.
But anyway, I was able to successfully login with alpha6, DIGEST-MD5 on igniterealtime.org, which runs Openfire (3.10.0 alpha). As far as I can tell, Smack behaves standard conform.