I'm playing Dragonfall Director's Cut V 2.0.9 and have to meet Frau Müller as shown in the PDA - but where to find?

I must admit that I forgot what meeting point we agreed on. At the statue in NO of Kreuzbasar is only the phone booth (Schockwellenreiter), the statue, der Weinkeller (Lucky Strike) and Simmy. In Cafe Cezve are only Altug and Luca Duerr waiting.

Bonus question: I want to clean the mess in the sewer system but have to talk to Frau Müller before. Is the order I solve the quest of any importance in this game?

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I just met her in the U-Bahn! This question can be ignored. SORRY!

Remains only my question about the order of the quests.

The order of quests is usually not impportant.
You will however get to a point where there's a big "Finish any outstanding business before taking this quest" warning.
Heed the warning.

The quests marked as optional may disappear at some point so I would give them high priority. Some of these optional quests also unlock additional purchase options in the vendors so they are worth completing early anyways. Furthermore, the optional quests tend to be a little easier overall than the main quests so you can use them to get a little karma and cash. The sewer quest however is perhaps one of the more difficult optional quests.