The systems page checks for reservation permissions to determine what messages to display on the "Provisioning" tab, but the permissions checks in the system model assume a logged in user. This can lead to an AttributeError and hence a 500 error when an anonymous user views the page (whether that will actually happen depends on various other things about the state of the system).
Sounds like a regression in 0.15.1.
On Gerrit: http://gerrit.beaker-project.org/#/c/2612
reproduced on my vm, updating to the latest packages(beaker-server-0.15.2-0.git.111.249da37.el6eng), 500 error fixed, actual results as same as expected results in comment7
Release announcement: https://lists.fedorahosted.org/pipermail/beaker-devel/2013-December/000923.html