Error while listing orders in backend

-- HikaShop version -- : 2.3.5
-- Joomla version -- : 3.3.6
-- Error-message(debug-mod must be tuned on) -- : DateTime::__construct() [datetime.--construct]: Failed to parse time string (19 Dezember 2014) at position 0 (1): Unexpected character

Hi,
I can't list my orders in the backend correctly. If I want to list some orders from last year, I'll get this error:

DateTime::__construct() [<a href='datetime.--construct'>datetime.--construct</a>]: Failed to parse time string (19 Dezember 2014) at position 0 (1): Unexpected character

I can get back ti the main backend page, but can't open my list of orders anymore.
Any ideas?
Thanks
Roger