The problem is known and could be narrowed down
Some Windows 10 versions have tried to install the Windows 10 Insider Beta version 1909 even though you are not Windows Insider. Since one is not an insider, the corresponding settings for suppression can not be used. The Registry Key or the Group Policy for the suppression of beta / insider versions also does not help.
This behavior seems to have arisen through a combination of Microsoft update actions. On the one hand, Microsoft has enabled a definition update of the WSUS server, which allows insider versions to be distributed to the WSUS. As a result, however, all clients querying the WSUS are automatically and without consultation with the insider updates served. Since the clients have no insider accounts, all countermeasures at the client DO NOT pull.
The settings on the WSUS have been changed so that now GARKEINE Insider versions can be delivered. That If you want to use insider versions, you must obtain them directly from Microsoft.
Solution for affected clients
For all those who have already installed the 1909 Insider Update, there is only the possibility of a rollback. However, since no abnormalities were detected on all our test computers, this step does NOT seem to be necessary. Only: Since 1909 was not yet officially released on 04.10, the distribution settings for the version 1909 for the WSUS are not yet available. That some updates (for example, cumulative Windows Update) for the 1909 release are not currently distributed through the WSUS. These can be obtained directly from Microsoft.
For all computers where the Insider feature update has not yet been installed, there is still the option to prevent installation. To do this follow the general instructions for preventing pending updates. For example, pureinfotech.com/delete-pending-updates-windows-10/, opens an external URL in a new window gives hints here (without any guarantee).