This problem has been solved by
Omar G.
You need to replace some lines of code in file "modcp.php"
Find :
if($unviewableforums && !is_super_admin($mybb->user['uid']))
{
$flist .= " AND fid NOT IN ({$unviewableforums})";
$tflist .= " AND t.fid NOT IN ({$unviewableforums})";
$unviewableforums = str_replace("'", '', $unviewableforums);
$unviewableforums = explode(',', $unviewableforums);
}
Replace with:
if($unviewableforums && !is_super_admin($mybb->user['uid']))
{
if(is_array($unviewableforums))
{
$unviewableforums = str_replace("'", '', implode(',', $unviewableforums));
}
$flist .= " AND fid NOT IN ({$unviewableforums})";
$tflist .= " AND t.fid NOT IN ({$unviewableforums})";
$unviewableforums = str_replace("'", '', $unviewableforums);
$unviewableforums = explode(',', $unviewableforums);
}
Once again, thanks to
Omar G. for solving this error