There are a lot of threads about this error 404 that comes with documents, links and download module, media won't show pictures any more, in short, all the urls containing linkclick.aspx don't work. I read them all, but when I reinstalled my sites, even deactivating compression, friendly urls and whitespace did not help. Now I found out the reason: There is a checkbox in IIS in the extension mapping properties page that is labeled 'verify that file exists'. If you check this box, at least for extension .aspx, linkclick stops to work. By default this box was checked in my installation, and when I unchecked it, all worked fine. You find that checkbox in IIS: YourWebsite - properties - basedirectory - configuration - double click on line with .aspx - uncheck the checkbox at the bottom.

Of cause all the other recommendations in this forum about compression, file synchronization and so on are valid too, this only is an additional feature that caused many hours of research for me.