I get this blasted error too. I have a BD30 running 1.3 firmware, connected through a brand-new Denon AVR-4308CI to a JVC DLA-RS1 projector. Most of the time this setup works fine. However, tonight the BD30 went into U73 error mode again.
The first time it happened I was watching the BD30, then switched the AVR over to my cable PVR (A Motorola DCT6412), then loaded a different disc into the BD30, then switched the AVR back to the BD30. I got the U73 error. I tried resetting individual pieces (BD30, AVR, Cable box, and proj) individually to no avail. I then powered down everything, and when I powered back up it worked okay. I thought it was an isolated incident.
Then tonight I turned on my system again and loaded a standard-def disc into the BD30, and again I got the U73 error. Now this time, I reset the whole system again, but the problem persisted. Then I reset everything again and it still persisted. I gave up and went away severely frustrated.
I just paid big bucks for the Denon so I could get HDMI switching and advanced audio format decoding. The latter works great, BTW. However I have to wonder if the Denon is causing the U73 problems now. I ran the BD30 right to the projector for a couple weeks prior and never had a problem. This HDMI stuff is really a poorly engineered pain in the arse, if you ask me.
Any thoughts out there?