After Robert provided the fix in https://wpforo.com/community/how-to-and-troubleshooting-2/viewing-profile-without-being-logged-in/#post-81303 (by changing the ID of the guest user to 4 - was 7 for some reason, not sure why ;O)

This in turn fixed the issue with the forums displaying

As such, marking this as now solved.

Thanks NutritionChat for your help too!