Re: ‘No input file specified’ errors
So, I figured this out myself. It turned out to be a problem with pretty permalinks. The bbPress instructions for enabling pretty permalinks say, in part:
2. Create a file called .htaccess in bbPress’ root directory. Put only the following line in that new file.
Options +MultiViews
3. Try it out. If it doesn’t work, your web server does not support MultiViews.
I suppose I was thrown off because for the most part, it did work. When I tried the documentation’s alternative instructions — feeding the output of rewrite-rules.php into .htaccess instead of the MultiViews call — the broken links suddenly worked fine.