It might well be an issue on Sky's Server.
However, if you are certain the issue is on your end, then clearing the SSL State in your browser, and cached data and site permissions etcetera, may resolve the issue.
Regarding SSL state, if you are using Edge to open the website, then:
- tap your Windows key, and type control panel then press enter key
- Select View by large / small icons
- Select Internet options
- Select the Content tab
- Click the Clear SSL State button