It could just be a failed installation of Zentyal. We have seen in our testing, on various virtual, and physical machines that Zentyal appears to install correctly, but then experiences odd instability, or loading issues. When we see this type of issue in the initial boot of Zentyal, we just blow away the installation, and try it again from scratch. The fact that you can get Zentyal to load in 20 minutes, makes this a painless fix to a glitchy installation image.
Kind regards,
Brain