There are some steps that you can try. I, however, get that error when I'm behind my company's firewall and proxy.
The engineers are working hard to resolve this issue so it should just be a matter of time. In addition to the suggestions made by Abambo, you can also switch to an incognito window and that should allow you to access the portal. We are sorry for the inconvenience and appreciate your patience.
Thanks a-lot, for now I just use a different browser and it is resolving the issue, for that website I use Internet Explorer.
Internet Explorer should not be used anymore. It will not progress and has been replaced by Edge. Firefox or Chrome or Opera will work also.
Technical stuff: Internet Explorer should be used only for those sites that have heavy ie-dependant programming. And those sites most probably are using some old Microsoft Sharepoint servers.