When browsing to http://omvIPaddress I received the below error:
OMV\Exception: Failed to read file ‘/var/cache/openmediavault/cache.omv\controlpanel\login_js.json’ (size=0). in /usr/share/php/openmediavault/json/file.inc:207
To resolve the issue I connected to the NAS using secure shell:
ssh root@ipaddressOfYourNAS and supplied the root password
After authenticating and connecting I ran omv-aptclean and rebooted the server for good measure.
I was able to access the web front end again after running omv-aptclean as per above.