G3 iBook strange LCD error. Variation of the logic board failure?

I've got a strange error with an old G3 iBook: The internal screen works fine in open firmware and during the first stage of the boot (Apple logo). However right after that (Apple logo with spinning gears) the internal LCD shows only a black screen. The backlight is still on, it just displays all black. However on the external display I have the normal OSX login screen. Could this be a variation of the logic board failure? Pressure on the GFX chip had no effect. I thought it might be a software issue and formated the internal drive, however now booting from the OSX install CD I have the same error: display ok till after the apple logo with the spinning gear, then it switches to the external display. I could not find any information on this error, am I really the first to experience it?
Any help you can give me is much appreciated.

Anyone? I'd really hate to have to thrash this one, since the internal LCD works in Open Firmware as long as I like and always blanks out at the exact same moment, which is rather unusual if it was a logic board failure.