AExpress won't join AExtreme network

I've had my AExpress joined to the main AExtreme wireless network for years for printing to a remote USB printer. I put it away for about a month, but just got a MacMini configured and wanted to use the AExpress to run remote speakers for iTunes.

Airport Utility (AU) would not recognize the Express until I did a hard boot. I configured it to join my existing network and everything appeared okay until AU rebooted the Express...it would not show up in the AU list and just flashed amber. I tried it a few times, doing both hard and factory resets, still no connection.

Decided to try and extend the network just to see if the Express would respond. Worked fine, lighted Green for go! I then went back into AU to switch it back to "join" vs "extend". Got the message "..sucessfully updated...restarting Express". Then I get the Message popsup "error...could not join network...". WTF. AU will not recognize the Express unless I do another hard reset. The AExtreme shows up fine and the connection is working.

I'm lost at this point. Anyone able to help? Thanks.

Airport Extreme is 5.7 firmware
Airport Express is 6.3