So a customer has a problem saying they can't download a specific file. I checked some other files, no problem.
Then I checked the specific file.. and indeed I get an error 400.

After some fiddling around I discovered that everything worked when I shortened the filename.
The generated URL was exactly 399 characters long. Removing 3 characters from the filename makes it work just fine.

Any idea what is causing this? Is there a limit somewhere in the DF HTTP Handler? I tried changing the maxUrl property in IIS but that doesn't fix it.