I don't remember running into this, but I don't think I've tried hitting ENTER during the direct access process (I've always gone straight to SET).

I think I know why I've never done this - it's because I'm using an MX-700 rather than the original remote. The only ENTER label I'm seeing on the original remote is a secondary label for the button whose primary label is MENU. Since I have that button mapped to MENU on my MX-700, I never think of it as an option. What's happening when you press ENTER during direct access is the 990 tries to go into the menu while the tuner has no station specified, so I wouldn't be surprised if that caused it to get confused and hang up. If it does affect other units (which seems possible, although I haven't tried it on mine yet), it would probably be an easy fix to cancel the direct access command and revert to the previous station when unrelated commands (such as MENU, an input command, a surround mode command, or other signals) are received.