GCal integration problem

Hello everyone!

First of all I would liek to say big thanks to developers of this wonderful plugin. It has so many essential functionalities needed for appointment calendar that I can't think of any better plugin built so far for this task. Seriously, well done guys!

However, one very important functionality without which I can not deliver this solution to the client is gCal integration. I have set everything follwong the instructions and double checked, but I get errors when I try test connection or import export option. I get blank page with follwings errors:

WARNING Error: [2] Cannot modify header information - headers already sent by (output started at /www/httpdocs/wp-includes/script-loader.php:835) /www/httpdocs/wp-includes/functions.php on line 888

WARNING Error: [2] Cannot modify header information - headers already sent by (output started at /www/httpdocs/wp-includes/script-loader.php:835) /www/httpdocs/wp-includes/pluggable.php on line 876

Do you have any ideas what might be wrong here?

Many thnaks in advance for your help!
Rento