Internal Links Broken

When an automatic link to a post is clicked, you get the following error:
“Bad Request
Your browser sent a request that this server could not understand.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.a 404 error”

Everything was working fine one day and then the next the links were broken. I’ve made no changes to my code. I upgraded to the latest version to see if that would fix but it hasn’t.

The auto links are in the form:

I’ve searched and only found fixes where someone changed code that broken the links.
Any help is most appreciated.