I bet it is the DX. I have a replacement DX and my original (with a little screen problem but otherwise functional). I updated one to Froyo clean, right out of the box from Verizon (after rooting). Market worked for about an hour and then stopped. The DX still on 2.1 reaches the market over WiFi without a problem.
I did a factory reset of the Froyo DX and after the reset it was still unable to reach the market (same network server error). After restoring the device with Titanium Backup, everything works except market.
I have also tried the AppBrain solution, downloading an app with appbrain and the market problem did not go away.
Other devices may have this problem, but i can say with some certainty that two identically configured devices running 2.1 and 2.2 behave very differently with the 2.1 device having no issues.
Lastly, if it helps anyone sort this out, immediately after the 2.2 update on the new DX the market didn't work, but that was solved by simply opening up the browser, closing the browser and going back to the market - so i chalked it up to a data connection issue.
I hope someone can figure this out and fix it since losing the market is not the best tradeoff for having froyo...
I did a factory reset of the Froyo DX and after the reset it was still unable to reach the market (same network server error). After restoring the device with Titanium Backup, everything works except market.
I have also tried the AppBrain solution, downloading an app with appbrain and the market problem did not go away.
Other devices may have this problem, but i can say with some certainty that two identically configured devices running 2.1 and 2.2 behave very differently with the 2.1 device having no issues.
Lastly, if it helps anyone sort this out, immediately after the 2.2 update on the new DX the market didn't work, but that was solved by simply opening up the browser, closing the browser and going back to the market - so i chalked it up to a data connection issue.
I hope someone can figure this out and fix it since losing the market is not the best tradeoff for having froyo...

