Which browser did you use to upgrade the FW?
I'm wondering if the FW load is corrupt on your unit somehow...
HT20/40 will grey out depending on your other settings, I'm not sure what exactly causes it, but I know when I was changing other settings like TKIP/AES/N mode/G mode/etc. that HT20/40 would be changeable one time, and not changeable the next.
For example, right now with G-only mode, channel 11, WPA2 only, AES, HT20/40 is greyed out.