Just to confirm, going to another topic and attempting to make that the new banner topic also fails? Does this occur in safe mode? Are there any errors in the browser DevTools? Also, what version is your forum currently on? I’m on v2026.3.0-latest.1 and I can’t repro this.
Then the banner is probably added by your theme or a component which is added to your theme, so I’d look there next. Is your forum public? Often, inspecting the element can help to find out what is responsible for showing the banner.