I don't think you need to reinstall the system. Maybe just OMV?? Before you do that, it would interesting to see your openmediavault-webgui_access.log and openmediavault-webgui_error.log in /var/log/apache2/.
The USB key might also be the problem. I am using a usb hard disk. What is the output of df?
Now that I look again, there shouldn't be an entry for omv-plugins on your system because I don't think you installed the omv-plugins.org plugin, did you? How about your output from dpkg-query --list openmediavault-*?