In short: it says that I was too blind to find it out myself
13:19:14: [0:40:38.482] OnlineKalender: Downloading: www.google.com/calendar/embed?src=<username>
13:19:16: [0:40:40.095] OnlineKalender: Result: (0) 200 OK.
13:19:16: [0:40:40.095] WARNING: OnlineKalender: Unable to parse the file. Are you sure it's a iCalendar file?
I used the HTTP-Link instead of the iCal Link 'cause Rainlendar said that you can add http/https/ftp URLs. So purely my fault! Sorry.
Btw, great product of yours! Got the link from a freind of mine and started translating/correcting the german version yesterday. Thanks for the great work!