As mentioned in the comments this question is already asked in German so this is only a translation or transcription resp. of the accepted answer to this question for the sake of completeness.
The term "meines Wissens nach" is simply wrong. The expression "meines Wissens" ("as far as I know") is used without the preposition "nach".
The preposition "nach" is used with similar terms like "meiner Meinung nach", "dem Vernehmen nach", "meinem Gefühl nach" or "seinem Urteil nach". And here nach requires the dative.
But the expressions "meines Wissens" or "meines Erachtens" (without "nach") use the genitive of "Wissen" and "Erachten" resp.
So harshly said "meines Wissens nach" is simply wrong.