I have the exact same issue. Nothing was changed on my system and I went to login and just get a blank page
ii openmediavault 2.2.14 all Open network attached storage solution
ii openmediavault-backup 1.0.11 all backup plugin for OpenMediaVault.
ii openmediavault-downloader 2.1 all OpenMediaVault downloader plugin
ii openmediavault-extplorer 1.3.1 all OpenMediaVault eXtplorer plugin
ii openmediavault-flashmemory 1.7 all fs2ram plugin for OpenMediaVault
ii openmediavault-iscsitarget 2.3 all OpenMediaVault iSCSI Enterprise Target plugin
ii openmediavault-keyring 0.4 all GnuPG archive keys of the OpenMediaVault archive
ii openmediavault-letsencrypt 2.4 all Generate free and valid SSL certificates for OMV
ii openmediavault-netatalk 2.0 all OpenMediaVault netatalk (AppleTalk Protocol Suite) plugin
ii openmediavault-nut 2.0 all OpenMediaVault Network UPS Tools (NUT) plugin
ii openmediavault-omvextrasorg 2.13.9 all OMV-Extras.org Package Repositories for OpenMediaVault
ii openmediavault-plexmediaserver 1.0.15 all OpenMediaVault Plex Media Server plugin
ii openmediavault-remoteshare 1.1 all remote share plugin for OpenMediaVault.
ii openmediavault-shellinabox 1.2 all Web-based SSH client.
ii openmediavault-snapraid 1.5 all snapraid plugin for OpenMediaVault.
ii openmediavault-usbbackup 2.2 all OpenMediaVault USB/eSATA backup plugin
ii openmediavault-virtualbox 1.3 all VirtualBox plugin for OpenMediaVault.
Everything is working correctly. This is extremely frustrating since nothing was changed, so there is no way to backtrack and the WebGUI not working makes the whole thing useless.