You will get the error you are reporting if the domain of the topic you are attempting to link to doesn’t match the domain of the Discourse URL you have entered on the plugin’s Connection Settings tab. Does this sound correct for your case?
No. We are on Communiteq (formerly DiscourseHosting).
Oddly I know this used to work. I don’t know when it stopped working, but the change must have been upgrading either Discourse itself or the Wordpress plugin. I’m pretty sure nothing else in settings changed.
I don’t think anything has changed in the plugin’s code for linking topics recently. Have there been any recent changes to your WordPress site? For example, have you moved it to new hosting, or changed the version of PHP that it’s using?
The domain name is parsed from the URL with wp_parse_url. It is possible that it’s not parsing the domain correctly.