Hallo Zusammen

komme zurück auf mein Problem mit dem Joystick-iMac weil ich in der Zwischenzeit die Lösung gefunden habe. Ich wollte es euch mitteilen falls es noch andere iMac Benützer hat, die sich mit dem selben Problem herumschlagen mussten.

This could be many different things. But, try this first.
After starting XP and your aircraft is loaded and ready, unplug your usb joystick that is not working and plug back in. Wait for a few seconds. That may fix this, but you may have to do this every time. If that fixes this, then try the following for a permamnent fix (dont ask me to explain why this works - but it just might work for you).
Go to OS System preferences. IN software update, uncheck automatically keep my mac up to date. Shut down your computer, restart and open XP. If your joystick now works then it should work every time - until you manually check for OS updates. Then you may have to unplug your usb joystick and plug back in to get it to work.
There has been a bug ever since about El Capitan on some macs that messes with usb attached devices whenever the computer does a check for updates (makes no sense, but this seems to fix this issue for most people that have it). ONly happens on certain models.
If this doesn't fix this. Then try removing any joystick software that you installed to manage or calibrate your joystick. Let XP manage all the calibration etc. itself directly.