You should log into the box via SSH or TTY and run one of the following commands:
omv-salt deploy run nginx (this module was marked dirty during the upgrade) or deploy the whole system with omv-salt stage run deploy.
After last update OMV 5 I cant get Web-gui with error: 502 bad gateway (nginx). Everything else seemed worked fine.After few days of strugling without forum you finally gave me solution! Second command of full deploy resolved my problem. Thanks once again. 
