Updating the FRITZ!Box 7490 from firmware version 6.30 to 6.52 causes the Aria scales to fail to establish a WIFI connection. No matter what you try the scales will not re associate with the router.
During my troubleshooting I stumbled across the cause, I had already fingered the firmware update to the Fritz!Box as the culprit and was just about to downgrade the firmware to the older 6.30 when found the following support notice.
Reading the KB article it was obvious what had caused the problem.
During the update procedure, the obsolete wireless standard 802.11b is automatically disabled in order to improve wireless performance in the 2.4 GHz frequency band.
The fitbit Aria scales just happen to be 802.11b.
To fix the problem is quite easy but the setting is hidden away in the Fritz!Box interface.
Now you are able to establish the wireless connection again.