BuddyPress.org

@geertbaven code might be written inside the template file of the theme
theme-name/buddypress/groups/single/home.php

You will need to remove bp_is_group_forum() code; you are not using forums that’s why it’s creating an issue.
Inside the latest version above code is removed, in core discussion developers had talked about it to put it back.

@geertbaven check following thread once

Groups and forum issue