Hi,
The DCNM Web UI uses port 443 by default, so make sure you use that specific that port (e.g. https://<dcnm-ip-address>:443).
Please keep in mind that your DCNM server could also be using a non-default port. In case you have access to the DCNM server, you can search for the "installer.properties" file (e.g. under C:\Program Files\Cisco Systems\dcm\dcnm\config) and look for the attribute "DCNM_SAN_WEB_PORT=".
I hope this helps.