I discovered the problem on this thread: If time-travellers always arrive naked how and what would they trade?
In the comments, another user posted a link to another part of the site. I followed the link.
Here is the worrying message I got:
Your connection is not private
Attackers might be trying to steal your information from meta.worldbuilding.stackexchange.com (for example, passwords, messages, or credit cards). NET::ERR_CERT_COMMON_NAME_INVALID
The link is:
What is happening? Is Stack Exchange compromised?