Have you checked your brush settings? Have you tried using Shape Dynamics in Brush Settings? Do you have any of the pressure options active in the options panel at the top of screen? Simply selecting a brush does not auto activate these pen/pressure sensitive options.
Thanks a lot for the support. Indeed pen pressure was active. Turning that off solved the issue. I´m really happy about that! A never had any problems of that kind with my Wacoms so I wasn´t thinking about it. Best, Manuel