Mandatory information about my setup:

Have I searched the tickets before posting? y
Have I searched the forum before posting? y
Have I read the documentation before posting (which pages?)? y
Joomla! version: 2.5.11
PHP version: (unknown)
MySQL version: (unknown)
Matukio version: 4.5.4 (using the old template)
Direct link to the page that shows the issue: http://www.crossfitontheroad.be/index.php/nl/de-lessen/je-les-reserveren


Description of my issue:

When members want to book an event they first ahev to type-in they username and password. Afterwards they can click on an event. This event shows then the details and the book button.They can book at that moment. That's working fine.
In case the members don't login in the beginning but first click an event and then try to click book, they receive an error message "Your booking was not successfull. Please enter a name and e-mail address". They're using the same (good working) login.

Thanks for help