What could cause my app to show a blank screen on startup after upgrading to iOS 4.2.1?

My app works fine on iOS 3.3, but after I upgraded my iPhone to iOS 4.2.1, I started to see some weird behaviour.

When I start my app on the upgraded iPhone, it goes blank. I have to tap the Home button to close the app.

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The problem is, the app did not crash. It just showed me a blank screen. So I don't have any crash report to track this issue down.

Has anyone seen this before? Is it an iOS 4.2-specific issue? What steps can I take to determine the cause so that I can fix my code?

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The behaviour of iOS did change between iOS3 and iOS4. As a first step, try walking through your code with the debugger. Also check your application:didFinishLaunchingWithOptions: method in your main App Delegate class.

A quick search also turned up some useful info about a similar issue: App shows white screen on startup after upgrading to iOS 4.2

The link also has info on how they managed to find out what was happening.