[Zope] strange behaviour:can not access source-view.gif but a copy of it

Hi there, in a plone site (but I see nothing plone related with my problem) I can not access an image with the id view-source.gif (it is used by kupu). when I copied it to the plone root the copy was named copy_of_view-source.gif all tough no prior object of that name existed. After renaming to view-source.gif and trying to access it trough its URL www.xxx.yy/view-source.gif nothing happens, only the watch is spinning to tell that the browser is still busy. if I copy that very same image to an other name, the copy can be accessed without problems. Can anybody give me a hint what the reason could be, or give me an idea how to hunt it down. thanks robert