I lost lot of PR on a site on mine, due to some changes in vbulletin old version to new version, I had my archive on following URL: http://forums.techarena.in/archive/index.php with a PR 5 this on vbulletin version 3.0x later I got forums upgraded to 3.5.x and archive was changed to new URL: http://forums.techarena.in/archive/index.php/ I lost all the PR from old page, archive was getting re-directed to the new page with /. Any comments ?
I doubt that the loss of PR has anything to do with the trailing "/". I used prchecked.com to check both versions of the URL and they both showed a PR of 0. Just a wild guess, but maybe you got hit with a duplicate content penalty because the archive is just a copy of what is already on the main site?
might be, there are 1000s of other sites using vbulletin and are not penalized for same. my main site, is still ranking with pr on internal pages as well...