i've just had the same on todays upgrade and the navigation menu is missing form the forum...
Version 4.1.12 which was todays upgrade.
Invision Power has replied to your support request.
Hi,
This is an issue with the Portal application you have installed, please contact the modification author and pass this error message along.
Declaration of IPS\portal\extensions\core\FrontNavigation\_Portal::children() should be compatible with IPS\core\FrontNavigation\_FrontNavigationAbstract::children($noStore = false)
Please let us know if you need any further assistance.
Regards,
Stuart Silvester
Invision Power Services, Inc.