Import CSV : Omeka encountered an error

Hi !
I am trying to import a very basic csv file and can't find why when I click the "next" button, the error message "Omeka has encountered an error" appears.

My titles are in the first row and are written like follows : "Dublin Core:Date";"Dublin Core:Title";"Dublin Core:Coverage";

Is there something I did wrong ?

Thanks and excuse my english...

Could you turn on errors messages - that might give us a better idea of what's going on. Thanks!

Thanks for the help!
For I'm not informatician, I can't understand everything but the error seems to be a zend error :

Zend_Session::start() - /web/omeka/application/libraries/Zend/Session.php(Line:477): Error #2 session_start(): Failed to decode session object. Session has been destroyed

Ok, so I only had a problem with a cell's format...
Thanks a lot !

(should I turn out errors messages, now ?)