After clarifying this with Mendix support it turned out that this was a bug.
The issue is solved in Mendix MTS versions 9.12.11 and 9.18.6.
Bernd,
I tried to reproduce your problem, and I couldn’t. Anyway, can you try the following :
1). Change the column of your Autonumber Attribute to Custom content
Drag a text box in the row and make it dynamic
As value give it: parseInteger($currentObject/<YourAttributeName>)
Either you get an error because <YourAttributeName> is a number and it expects a String, or it works.
In case of Error: You found a bug (maybe upgrading to a higher version is an option. Also look into release notes to see if it is a known error)
Go Make It