Now you mention this… I previously couldn’t add unicyclist.com to the app with the same error.
When I tried again after a few weeks, I could add the forum to the app.
I didn’t understand why it was repaired, but reading your message I remember I changed my CDN from Stackpath to Bunny.
Note that I use Bunny’s default settings. Nothing special.
So, this issue may come from a CDN configuration, I guess. Not saying this is the case for you, of course.