I have got exactly the same problem. The update from 1.00_1.26 to 1.52_1.22 fails every time with message:
Now re-starting!
Firmware Upgrade Fail!
Estimated time to restart about 60s.
Please Wait... 60s. later, if the page does not automatically refresh, click here.
I have tried updating from Linux + Firefox (no firewalls) and from Windows + IE (disabled antivirus software). I have pushed the reset button at the back of the router first. I have tried updating from WLAN side and from Ethernet side. I have downloaded the firmware several times during the last three days. I have also updated with the same version (1.00_1.26 over 1.00_1.26) and it updated with success. But still the update to 1.52 fails every time.
With best regards from Finland,
-make