I am using the standard English Layout on my iPad and Mac (US keyboard layout). Mac is set to English as well as Photoshop.
Now that you mention the default keys in Photoshop, this was the problem. I actually had no shortcut assigned to "Undo/Redo" in PS. I use the "Step Backward" and "Step Forward" functions instead because they do what I want which is going back a step every time I press it. So now I just assigned a keyboard shortcut to the "real" undo function in PS and the same in Astropad Studio and it works now. Though, I will never use that gesture if it works like this then, because I want to have the "Step Backward" function when I tap with two fingers. I usually want to go back in History several strokes, not just one. Can you let us just assign any key to that gesture? It also wouldn't make sense to have a "Undo" and "Redo" gesture in Astropad for Photoshop because it's the very same function in PS. So you have the same function on two different gestures now.