The "online today" setting is one of four that don't appear to have been installed correctly.
modules/ForumsPro/admin/config.inc
* Notice line 75: Undefined index: sig_groups
* Notice line 84: Undefined index: custom_fields
* Notice line 265: Undefined index: online_today_enable
* Notice line 294: Undefined index: guest_secimg_enable
Some time ago I placed a request on the DF forums for people to feedback whether they had any issues with those particular fields and got none reported »
dragonflycms.org/Forum...42/#146042
The ForumsPro upgrade path is quite messy and obviously the version left in the MwA repository was simply incomplete.
As I've previously stated, I'm in two minds at the moment, and I'm tempted to cease work on ForumsPro and try to integrate it into ForumsPlus, a version more compatible with the original CPG-BB forums.