[TYPO3-seminars] Error when trying to create speakers or places in FE

[Christian Hennecke]

Hi,

When I press the button to create a speaker or place in the FE form for 
creating or editing an event, I get the following error message:

Fatal error:  tx_rdtmodalbox::majixShowFreshBox() [<a 
href='tx-rdtmodalbox.majixshowfreshbox'>tx-rdtmodalbox.majixshowfreshbox</a>]: 
The script tried to execute a method or access a property of an 
incomplete object. Please ensure that the class definition 
"tx_dhdbmm" of the object you are trying to operate on 
was loaded _before_ unserialize() gets called or provide a __autoload() 
function to load the class definition  in 
/var/www/xyz/htdocs/typo3conf/ext/ameos_formidable/api/base/rdt_modalbox/api/class.tx_rdtmodalbox.php 
on line 29

Any ideas what the problem could be?

Thanks,

Christian