Check that the services are running and ensure that VeeamDCS.exe process is climbing for memory usage.  You might need to give it slightly more time to get going as I have seen this as well then just wait a bit and it works.

thanks Chris. I will check this but Veeam processes have been running for some time. The VONE client works ok. I might bring down Veeam completely, restart then check the process you mentioned and make sure its consuming memory. 

Hope it helps.  I know this can be finicky so give it time to get going before trying to log in to the web UI.  

thanks Chris, will keep you posted….

I already had this problem in the past.
In my case was the veeam one came with an express SQL that only supports 10 GB of data. So i needed to connect in the Veeam one server click in the icon of Veeam One Client > Settings > Server Settings > Other > Support Utility > Click Launch > General> Retention Policy Period > Change the policy to a lower numb and apply retention.
Note: If you change from 1 year retention to 3 Months you’ll only have 3 months of history.

looks like since v12 upgrade, the port has changed to 2741. 

https://VEEAMOneWebClientFQDN:2741

this solved the issue

Glad to hear you resolved the issue.