If I where to hasard a guess at what is happening you are experiencing a bad version of the FW bug that hexparrot is battling. (more info here: A funny thing happened on the way to my MineOS WebUI)
However, this is just an idea, and not confirmed. To do that we need to have a look in the logs that are mentioned above.
I also reccomend trying to upgrade the WebUI to the latest release with the commands given above.
If you do not wish to wait around for logs to be read, and workarounds to be found you can go down another path:
Make another VM for a new MineOS instance, but do not use the ISO. Instead go to http://releases.ubuntu.com/14.04.4/ and release the correct 64 bit ubuntu server (the server ISO’s are listed at the bottom). This will install a basic ubuntu server.
Then follow HOW TO: install MINEOS-NODE on UBUNTU SERVER 14.04.2 to install mineos on your ubuntu server.