Hey there,
I've sorted it out. I opened every hidden support option and compared them. There are a few more that are influenced by changing the Extruder which I didn't notice at first.
The problem was caused by "Support Stair Step Height". While it's 0.3 for Extruder 1 it turns to 0.15 for Extruder 2 and causes the gaps. I don't really understand why. Every other value (0, 0.3, 0.6, 0.9) seems to work fine.
Anyhow it works for now 🙂