I started the app on my laptop without external screens. So it was displayed properly on the integrated screen (3840 x 2400).
I put the laptop into sleep mode, connected it to my docking station which has to external screens (2560 x 1440 each). The main display is one of the external monitors.
"Waking up" the laptop opened WorkingHours on the main screen like it is shown on this screenshot.
Whithout maximized window it look like this
Moving the window back to the laptop screen fixes the problem.