This bug has been around since about 2005, people come on forums and report that their client, which was working perfectly for months, all of a sudden only returns this error. They can't upload, download or report work anymore.
Most of the times we try to help in forums, but since none of us knows what is causing it, eventually the person is left behind without help. As far as I know, the only real fix is to reformat, reinstall the OS and reinstall BOINC clean. Which isn't good enough, imho.
So I am inputting it into Trac, as a blocker.
Old threads it happened in:
http://setiathome.berkeley.edu/forum_thread.php?id=45585#718991
http://setiathome.berkeley.edu/forum_thread.php?id=46842#747721
http://www.primegrid.com/orig/forum_thread.php?id=304
http://einstein.phys.uwm.edu/forum_thread.php?id=5830
http://www.bjoernhenke.de/forum/viewtopic.php?t=433
http://boinc.gorlaeus.net/forum_thread.php?id=109
http://boinc.bakerlab.org/rosetta/forum_thread.php?id=4039&nowrap=true#52221
http://boinc.berkeley.edu/dev/forum_thread.php?id=2584#16465
The latest thread (and person I am trying to help):
http://einstein.phys.uwm.edu/forum_thread.php?id=6729
It would be nice to have a final answer to this problem.