Hi @joerg2503,
Thanks for giving that a try, let’s look at a couple of other options:
1. Please install and activate this plugin:
https://github.com/automattic/jetpack-addons/archive/add/jp-rm-options.zip
Once activated, it will remove any stale Jetpack data on your site. Deactivate the plugin, then try to connect again.
2. If that doesn’t work, please delete that plugin and then try installing and activating this one instead:
https://github.com/Automattic/jetpack-addons/archive/add/old-connection-flow.zip
Once installed, try to connect again. This plugin uses a slightly different connection flow, which may help with the issue you’re having. If it works, the plugin can then be deleted.
If neither of those work, delete both of those plugins and come back to us so we can look into this further.