I hit the same case when the first time update the firmware 1.21....then I fall back to 1.11, and then re-upgrade to 1.21 again...problem gone and so far so good.
So, I think it might be caused by the upgrade program or corrupted data.
Suggest :
1. re-download the latest 1.21 version
2. do a complete reset of the router
3. re-fresh the firmware
4. do a compete reset again
5. manual setup (do not reload the configuration file)
Try and good luck.