have an xml with something like :

<ymin_item_key>:</ymin_item_key>
<ymax_item_key>:</ymax_item_key>

importing it fails with :

Missing item [:] for host [Template_Windows2]

also debug shows frontend doing :

SELECT DISTINCT i.* FROM items i WHERE i.key_=''

1. reported error should explain which value for which graph is incorrect;
2. if we know that item key is not valid (neither single colon nor empty key is), we should not do useless sql queries with it