What release of XMM are you speaking? the new 5.x?
When this problem happens exactly? In the main window (quickedit)? or in the Edit Window?
I have make some tests and seem to be OK here. I have try with an existing movie (that has already pictures) and with a new movies (without pictures) and seem to be OK.
It seems to work fine now... I don't know what happened before. Maybe I did something wrong. I have another question about covers.
When exporting with movie cards, I would like to choose Cover destination folder. Is it possible sometime in the future? I would like to store covers in the same folder as html.