Hannig wrote:Dies und andere Besonderheiten der Voynichsprache deuteten auf das Aschkenasische hin, eine lokale Variante des Hebräischen, die unter anderem in Südrussland gesprochen wurde.
I think he is referring to Yiddish. Aschkenasische is a more general term (at least it is in English). Yiddish was spoken in numerous Jewish communities in south and east Germany (more properly called Bohemia or southeast Holy Roman Empire at the time), not just Russia.
If he is saying it's Yiddish, that's linguistically different from Hebrew. Yiddish has a significant infusion of German loanwords and German grammar, even in the 12th century. It's not Hebrew in the traditional sense.
Here is a famous Yiddish example from the Worms Mahzor:
“Gut taq im betage se vaer dis mahsor in beith hakenseth trage,”
Gut taq = Guten tag; dis mahsor = dieser Mahzor, etc.
Yiddish is frequently more German than Hebrew.