Hello Michael,
I managed to solve the strange error message by re-creating my test project in a different version of AS and choosing a higher version of the ACP10 Motion software (from 3.14.3 to 5.03.3). Now a simple safety program created according your your (excellent 
) recommendations can be compiled and is ready to be downloaded to the X20SL8100 module.
BUT, unfortunately, I spent many hours trying various things yet I failed to connect to the SL device from the SafeDESIGNER software!
So, I have created a separate thread about this issue and after getting help from @alexander.hefner, I think I know why that happens.
If you have more information on that subject, please reply there. I would really appreciate that!
Thank you and Best Regards,
Alexander