This error has been driving me nuts. We have a server running Apache and Tomcat, serving multiple different sites. Normally the server runs fine, but sometimes an error happ
Although you did mention mod_cache was not enabled in your setup, for others who may have encountered the same issue with mod_cache enabled (even on static contents), the solution is to make sure the following directive is enabled on the Set-Cookie HTTP header:
CacheIgnoreHeaders Set-Cookie
The reason being mod_cache will cache the Set-Cookie header that may get served to other users. This would then leak session ID from the user who last filled the cache to another.