Tearing my hair out over http error 0 ...

I am running a Drupal multisite installation on a LAMP server running RHEL 5. The original installation was an Acquia build, and we recently upgraded the core to Drupal 6.14 equivalent. The problem now is this: I cannot upload files to one of my sites without getting a "http error 0 /site/upload/js" message. This happens with both the core upload module as well as the filefield module, and this was not an issue prior to the update.

However - it only occurs on one of my sites, and I am able to upload files successfully on my other sites. Additionally, I am able to upload images to my affected site, but not PDFs (the native format that my clients use). The modular profiles on the two sites differ by about 10-15%, so I can't isolate it to a single difference. In any case, this is a production site so I can't start turning modules on and off randomly to test it out.

I have seen any number of threads here and there about this error, and it seems to be that there are multiple multiple potential causes. I have updated jQuery, played with the caching, checked the permissions on the file system, and read through too many discussion threads to keep track of.

I can say this: