g7net please do not dig multi-years-old discussions. The problem was in an older Flarum version and has since either been resolved or become irrelevant.
Also your URL format doesn't seem correct. It shouldn't contain a trailing slash.
When installing Flarum according to the official instructions, that value should already be populated with a sensible value. It's best to configure HTTPS + customize paths prior to installation to prevent issues with the config url.
