I installed 2.8 on Centos 6.5 following alongside the Ubuntu 12.04 install. This seems to work well with one exception. The practice accounts are used sequentially through practice9, then there is the error "Your authentication failed. Please try again. Please speak with your instructor if you need help." when these are used up. This eventually resets after some time. Rebooting the machine resets these. Restarting mysqld and httpd does not reset these. Regular accounts work well, only a problem with the practice guest login accounts.
A solution is to create more practice accounts, but I wonder if this is a flaw in configuration which applies only to the practice guest login accounts.
I'm pretty sure that this has always worked like this. The practice accounts log themselves out after 30 minutes of inactivity but not sooner. This allows people to use a practice account without being booted off by someone else trying to sign up for a practice account and using the same login name. The timeout length is determined I believe by
# $sessionKeyTimeout defines seconds of inactivity before a key expires
$sessionKeyTimeout = 60*30;
in the file conf/defaults.config
You could override it in localOverrides.conf
Hope this helps.