[Plugin: No Flash Uploader] After install, site gives me 500 Internal Errors

So, like so many others, I have been having the infamous “HTTP ERROR” during crunching pictures, so I wanted to give this uploader a shot until our fearless leader Matt provides a 2.5.2 update. ?? I have faith that he will!

But now, of course without flash and the old uploader instead, the site pauses for a few minutes (this is with a large image, about 2MB, and these always used to work on my site) and then the little window-in-the-window turns white and gives me a 500 Internal Server Error.

Any thoughts?