safari problems with Natwest login

I have had problems logging in to my natwest online banking since upgrade to Sierra. I get the following error message

"Safari can't open the page "https:/nwolb.com/default.asap" because the server unexpectedly dropped the connection. This sometimes occurs when the server is busy. Wait for a few minutes and then try again"

No success with retry If a restart my iMac the first login is successful but subsequently the identical problem happens. I have allowed cookies and have no restrictions on my searches All my other searches are OK and I can login with Firefox or Google Chrome. Any suggestions what the problem might be?