Petervdv 🏳️🌈 As Bernhard stated, the "neutral" Variant would be "Sehr geehrtes", but this is rather unusual and may look offensive.
You would have to write something like "Sehr geehrtes bunq-Mitglied" ("Dear bunq-Member"), but that is kinda unpersonal. You can't even write "Dear Customer", because that would be "Sehr geehrter Kunde"/"Sehr geehrte Kundin".
It's really hard to do this right in german, as the Gender is a pretty important thing in our Language.