Most commonly this is down to a WP theme doing non-standard things.
Go to the forum admin > Integration > Page and permalink panel. At the bottom, in the Theme Display Options. Try turning one of the top two items on – then the other and maybe even both if necessary to see if one of those resolves it.

Turning off the “Allow multiple loading of forum content” and turning the “Limit forum display to within WP loop” option on, did the trick. Thank you!

excellent.

np. glad we could help!