First, that's not an iPod 2G, but an iPod Nano
2G. It's important to write the full model name, as iPod 2G is another model.
If you want to completely uninstall Rockbox you need to uninstall the bootloader too like you're trying to, or it will keep on searching for rockbox.ipod which doesn't exist anymore since you uninstalled Rockbox files (but the bootloader still remains). That's why it returns that rockbox.ipod error.
I think that Rockbox Utility can also uninstall the bootloader, just search for the button that does it. If I recall correctly (and probably I'm not recalling correctly), it's below the button that you clicked to uninstall Rockbox files.
If you could, however, not uninstall Rockbox for a while until we further investigate your problem... if you don't mind, of course. I have it installed on my iPod and it's wonderful so it'd be nice if you could also get it working. It's just that some days ago another user with a problem similar to yours appeared on the forums, and the solution that worked for him/her might be the same for you, if my bet is correct... I just need to confirm with someone better experienced than me
For Buschel: do you think this is another case like the guy with a white screen on his iPod at http://forums.rockbox.org/index.php/topic,27777.0.html
? LCD undervolting problems again?
If yes, do you have any idea why some screens of the same type work at a certain voltage on some iPods (like mine and yours), and others don't (strange effects on the screen)?