Hi,
I am not sure if this is a problem of og_forum module or OGUR. I have several groups in my website. Each of them has its own forum through og_forum module. There is a "group manager" role on the site with "administer comments" and "administer forums" permissions. Each group has a user with "group manager" role granted by OGUR. "aminister forum" permission works perfectly but "administer comments" doesn't work in the forums section.