I have the same problem too.
After a reboot of the router every thing workes fine, but some minutes or hours later it does not.
A Windows XP PC says nothing, just tries to reconnect.
A Samsung Galays S3 mini says authentication fault.
A Samsung Galays S3 says authentication fault.
The router is brand new (1 week in use) and the issue exists since the first day.
I have a Fritz!Box router with WLAN and DECT enabled in an other room separated by a concrete wall, but visible of course to all devices. The Fritz!Box acts as DSL modem, the DLINK is just an access point at the moment.
Any new info on that topic?