We're aware of the problem, and know what's causing it, but it's not code of ours, so it'll take a little longer to get fixed.
The problem is mod_backhand and how it deals with multiple cookie headers in HTTP requests and responses. mod_backhand is the software that manages the load between the multiple servers.
If anybody wants to fix the mod_backhand code, let me know. Otherwise I'm going to work around their bug, or see if its maintainer has time to fix it.
One way to fix this would be to turn all but the one original web server off, but then the site would be slow. We can live with this little problem for a day or so.