@[email protected]: Resaving permalinks did the trick. I first changed permalink of course to course then saved, and then changed again back to activities and saved permalinks.
For others facing the same issues :
Please re-save your permalinks.
Most of the users facing and reporting this issue are not following the notification message posted at the very top of forums.
I've not been able to figure out why the issue is occurring in some installations.
Here's the Temporary Fix.
Add this code in Theme - Functions.php
add_filter('wplms_course_nav_menu','wplms_course_temporary_permalinks_fix',99);
function wplms_course_temporary_permalinks_fix($nav){
if(!empty($nav['curriculum']))
$nav['curriculum']['link'] = '?';
if(!empty($nav['activity']))
$nav['activity']['link'] = '?';
if(!empty($nav['drive']))
$nav['drive']['link'] = '?';
if(!empty($nav['members']))
$nav['members']['link'] = '?';
if(!empty($nav['admin']))
$nav['admin']['link'] = '?';
return $nav;
}