While it is not the constitution, but the criminal code, being a nazi without at least a basic veil of decency will get you jailed for publicly voicing your beliefs:
Whosoever, in a manner capable of disturbing the public peace:
incites hatred against a national, racial, religious group or a group defined by their ethnic origins, against segments of the population or individuals because of their belonging to one of the aforementioned groups or segments of the population or calls for violent or arbitrary measures against them; or
assaults the human dignity of others by insulting, maliciously maligning an aforementioned group, segments of the population or individuals because of their belonging to one of the aforementioned groups or segments of the population, or defaming segments of the population,
shall be liable to imprisonment from three months to five years.
Well, in theory you can also go to jail, for saying you think the catholic church is a organisation of criminal pedophiles. (blasphemie paragraph)
Does not happen in reality, though.
There are Nazis in jail though for denying the holocaust (Horst Mahler) and for trying to incite violence (Landser). Not for expressing their nazi ideology, but for direct calls for murder.
Plenty of NDP functionaries (and some AfD too) were fined or are on parole for Volksverhetzung. Just because it doesn't usually escalate into jail time doesn't mean the law is not being applied.[1]
What you were referencing is §86 and §86a StGB.