[WetWired]

Well, the link isn't showing because your navbar template doesn't have the template hook for it. The link is added to $template_hook[navbar_buttons_left]. Assuming your navbar changes are just new/rearanged links, you should revert your navbar and redo your changes, being carefull not to remove any template hooks, since this will affect other mods you might install, too.

As for the page itself, that situation is most odd. The only thing I can think of is that some other mod is changing $_REQUEST['do'] or also uses the forumhome_start plugin and is returning. I suggest turning off all other mods and see if the problem persists. If not, turn them on one by one untill the problem comes back; the one you just turned on when it stops working is probably the problem.
Depending on the cause, you may be able to work arround the problem by going to Plugins&Products>Plugin Manager, finding the Activity Modification forumhome_start plugin, editing it and setting the execution order to 1.