Trouble updating from free -theme

Greetings! I was running the Catch Kathmandu free theme so far and decided to upgrade to the pro version. I bought it and downloaded it to my PC. Then I went to admin my website and added the theme by selecting the downloaded ZIP folder, but whenever I activate it the whole site goes blank!! It re-appears when switching to the free version again and it also keeps on working when I switch to one of the other free themes that show up in my themes section. This damned pro version however doesn't work... This seems quite weird to me, especially as I am just learning to build a word press website, so I hope that someone can help me here. The URL is www.42dreams-photography.de Thanks in advance Carsten