Folding@Home is, as so many times before, broken on Linux systems with a reasonably up-to-date version of libc and either segfaults or stops working with bizzare error messages such as 'Couldn't CosmHTTPOpen' or 'Cannot get ID from server'. Since many solutions posted in the interwebs are wrong or deprecated, here's a viable workaround which I know to work on Debian, Ubuntu, Mandriva, and Arch (and Fedora, I've heard).
Install nscd, then check your /etc/nscd.conf with your $EDITOR. Find the following line:
enable-cache hosts no
Change "no" to "yes", if necessary, then save and exit the editor. Finally, issue a
service nscd restart
and fold away.