custom field won’t save

i don’t get it: i use the “custom field” (key=”pdf_file”) to specify the path to a pdf file. for one post, it saves the url and displays it correctly in the blog frontend, but when i try entering it on other posts (either selecting the “pdf_file” field from the select menu or typing it in the textfield) and hitting on “save post” it disappears from the post editing interface, although the status message says “custom field updated”.

what am i doing wrong ?