thanks for the help in advance!
I've posted some events and when i hit the category page for the first time the events are showing up just fine.
When i now visit e.g. click on category or venue or refresh that page the following error appears where the date / time normally should be.
Warning: DateTime::format() [function.DateTime-format]: The DateTime object has not been correctly initialized by its constructor in /var/www/vhosts/cave-con.de/httpdocs/wp-content/plugins/event-organiser/includes/event-organiser-utility-functions.php on line 35
visit http://cavecon.de/events/venues/saal for a working example ;)