You probably have pop-ups disabled in your browser. Your browser may display a message that a popup has been blocked and offer to open it later. However, opening the popup later causes an error in Solar Planit.
To fix the problem, you have to allow popups in general or you have to create an exception for the page.
Add
http://www.solar-planit.de. You can read how to do this on the pages of the respective browser manufacturer:
Firefox
Google Chrome
Internet
Explorer
Safari
Opera