Hi PeterAxel,
Thanks for posting in Apple Support Communities. From your description it sounds like you can connect the Smart Keyboard, but it disconnects. The article you used to post to us was a great resource. Let's try the steps below, in the exact order to see if this helps.
Found in Use your Smart Keyboard Folio or Smart Keyboard with your iPad - Apple Support, try this:
Get help
If your iPad doesn't detect your Smart Keyboard Folio or Smart Keyboard or you see an "Accessory not supported" alert on your iPad, make sure there's no debris or plastic covering on the Smart Connector pins on the keyboard or the Smart Connector on the iPad. Also, try these steps:
1. Make sure that your iPad and Smart Keyboard Folio or Smart Keyboard are in the type position and that you have the latest version of iOS or iPadOS.
2. Remove the Smart Keyboard Folio or Smart Keyboard from your iPad.
3. Restart your iPad.
4. Reconnect the Smart Keyboard Folio or Smart Keyboard.
That should help with resolving the issue with your iPad and Smart Keyboard. If not, we recommend contacting Apple Support by going to Get Support.
Take care!