The site is fixed and back to life. Thanks to Tim for granting me access. Gory details below. I haven't check all links, so it may not all work. If you find something that doesn't, let us know.
-hilmar
It turns out I was wrong about it using WordPress. Instead it uses a custom-installed Typo3 (http://typo3.com) as the CMS. The installation was referring to a location for web-application file storage that does no longer exist. I created the necessary file structure at the new location, and restored the content from a backup I found among the website files, which was apparently taken Apr 7, 2008.
SourceForge backed up the files from the old webserver file storage location, but at a location that's not accessible through the normal shell access. Due to the nested directory structure I would need to use something like rsync to pull it down recursively, and if we encounter problems with the restore from backup I can look into that. However, looking around there and spot checking the files seemed to be all of the same date and size as in the backup I used for restoring, so maybe we're good already in terms of having the latest version before it all went broke. BTW this must have happened on Sep 18, 2008; I guess the site was broken for almost a year ...
SourceForge now provides hosted apps that are customizable but maintained by Sf.net. Not surprisingly, they advise against installing your own, as projects have to maintain and upgrade those themselves. I have enabled MediaWiki and WordPress but there are other options too (https://sourceforge.net/apps/trac/sourceforge/wiki/Hosted%20Apps ). Sooner or later the content should be probably migrated away from the custom Typo3 installation to one of these hosted applications, unless someone knows of a specific advantage of sticking with Typo3.