It's their SSL certificate. Their root domain is ok, but their forums subdomain is not.
I tested their website from three different operating systems (Mac, Linux, and Windows) and four different browsers. I tested from multiple locations around the world.
Their current certificate setup works as of 29 July 2021, but their certificate configuration is confusing to me.
The home page:
https://www.woodnet.net
... is secured with a Let's Encrypt certificate. That certificate was issued two days ago, 27 July 2021.
Let's Encrypt certificates are free, but they are good only for 90 days. A typical installation has a setup where the Let's Encrypt certificate is automatically renewed. I wonder whether they have their Let's Encrypt certificate management configured correctly and working properly.
Subject Names: forums.woodnet.net, woodnet.net,
www.forums.woodnet.net, and
www.woodnet.net
The forums page (and the pages below that with threads, etc.):
https://forums.woodnet.net
... are secured with an Amazon certificate issued 22 January 2021.
Subject Names: woodnet.net, and *.woodnet.net (a wildcard)
I have never visited the site until today, but I noticed old threads from 2020 about certificate expiration issues. One thread was dated 2 February 2020, and another thread dated 3 May 2020. That's exactly 90 days apart, so circumstantial evidence suggests that they have issues with Let's Encrypt certificate renewal. Just a hunch.
The website is hosted through multiple Amazon servers. As of this writing:
woodnet.net and
www.woodnet.net are hosted at 23.23.51.128 in Virginia, USA
forums.woodnet.net is hosted at 13.226.214.4, 13.226.214.116, 13.226.214.38, and 13.226.214.9 in California, USA