Same result here on a Airport Express 802.11n (A1264). Basic config is to extend the (hidden) Wifi network from a brand new TC (firmware 7.7.1) with MAC address filtering (no Airplay activated). On firmware 7.6.3 everything at the Express was working as designed. After upgrade the to 7.6.4: for some minutes everything working like charm. After that time, no connection anymore - the Express is no longer extending the Wifi - and the ugly thing: the Express seems to look correct from client side (is also green LED) - but when the client is trying to connect, nothing happens - but: as long as the Express is powered on, the client wants to connect to it (which doesn't works). After poweroff/poweron of the Express: everything is working again for some minutes - after that, same issues as described. After rolling back to firmware to 7.6.3, everything is working as designed again - no issues since then.
And: on new Express (802.11n 2nd gen (A1392) with AirPlay activated and in network extending mode): after upgrading to firmware 7.6.4 music drop outs occuring after some minutes of playing content (via Airplay from a iphone 4). First, some drop outs...then stop of playing music at all - i had to reconnect airplay (choose internal iphone speaker, hit play, and then choose AirPlay device again and hit play again)....but: networking extending was still working as designed all the time. After downgrade to firmware 7.6.3, everything is working as designed again....no issues.
conclusion: in my opinion, firmware 7.6.4 has some general problems on different device types and using old firmware version 7.6.3 produces less issues.
BTW: i didn't tried that much with: reset back to factory defaults and rebooting and reconfiguring the devices and trying again because in my opinion: the piece of hardware simply has to work stable and reliable without playing around for hours and hours - if i would like to spend hours and hours in not working hardware, i wouldn't have bought devices from Apple.