I have no idea either ... *shrug*
Only thing I could think of to recommend is check your httpd.conf file, and verify that all paths either end with a / or don't ... I just browsed my conf file and double-checked, and none of the paths that I have set up end with a trailing backslash.
Only reason that I would even think of recommending this is that it seems to me like when you don't add the trailing backslash, Apache is looking for a file of sorts, and is crashing out on you. Adding the backslash forces Apache into a directory, where it can then serve up the default file of choice in your configuration.
As I type this, I just had another thought: Have you checked the Apache logs after you've received one of these messages? It may / may not shed some light on the problem you are experiencing.
HTH
Greg