No problems with any other sites. The site is not down when I check using sites such as "Down for everyone or just me", "Is it down right now" etc. They all show the site is up. I also emailed the site administrator and he is not aware of any problems.
The problem seems to involve Swisscom as the site opens normally on my mobile phone using mobile data on the Salt network, but not when using Wi-Fi on my Swisscom internet connection at home. Same problem with both Windows 10 laptop and Android tablet. It times out after a minute or so. Same with 4 browsers. Tried deleting cookies, cache, history etc. No change.
I spent about 20 minutes with the technical support number today (0800 800 800). The person I spoke to was very helpful and tried rebooting the router, changing DNS settings on my laptop etc., but it made no difference and the site still will not open. He could not suggest any further action that might help.