The columns can be resized using the header row (Einzelheiten, Bezeichnung). If you enable row header scrolling (https://doc.daypilot.org/scheduler/row-header-scrolling/) the Scheduler will display a splitter between the row headers and the main grid.
This splitter doesn't change the column width, only the total row header width. It looks like the splitter masks the handle that resizes the last column which is something that could be fixed.
However, if you don't need the row header scrolling the best solution is to turn it off. Then it will be possible to resize all columns freely.