Wasn't there once a predefined action "go/switch to last/previous app" (or something). Or is it just my imagination, anyway I can't find it anymore. I know I could use cmd+tab, but that doesn't work well in a macro because I don't use the macos to switch between apps. I try to replace the red action.
I think you might remember this from the "Send shortcut to specific app" action, which let's you switch back to the app that was active before sending the shortcut - but that won't help in this case.
I could easily add this action though, as BTT always keeps track of the previous active app.
Yes, that was exactly it!
Das wäre wunderbar. Vielen Dank, Andreas 
Sorry, is there any news here?
yep, I think you can use the new "Activate Previous App / Window" action.
Yes, works perfectly, thanks a lot!