Safari can't open the page because the address is invalid query
I am trying to send information to the Portuguese tax office. The first page asks for all the details, no problem, then there is a continue button which takes you to a page where you can add documentary proof of the details entered, pdf files etc. When I click on this I get the message "Safari can't open the page because the address is invalid". The web site is legitimate, it is the Portuguese tax portal, and I have cleared the cache in Safari, as well as disabling extensions. This is on a Mac mini 2012 - not on your list of Macs so had to choose later model, running Catalina 10.15.7. Is there anything else I can do to overcome the problem. Incidentally I have tried using the same site in Brave and Chrome, and each time I am redirected to Safari which of course doesn't work.
Thanks in advance
Mac mini 2018 or later