Twain and TwainBridge problems with Canon MP800 scanner and Photoshop CS

I just bought a Canon Pixma MP800 multifunctioning printer/scanner/copy machine which works otherwise fine except that the scanner doesn't appear on my Mac, the MP Navigator that came with the machine doesn't launch and vice versa, the scanner mode on MP800 doesn't lauch the MP Navigator.

I conclude that there's a serious communication problem with the two machines due to Twain and/or TwainBridge as I located the following two crash reports:
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Command: MP Navigator
Path: /Applications/MP Navigator 2.0.app/Contents/Resources/MP Navigator.app/Contents/MacOS/MP Navigator
Version: ??? (???)
PID: 442
Thread: Unknown

Link (dyld) error:

dyld: /Applications/MP Navigator 2.0.app/Contents/Resources/MP Navigator.app/Contents/MacOS/MP Navigator truncated or malformed library: /System/Library/Frameworks/TWAIN.framework/Versions/A/TWAIN (file is empty)
----
Command: TWAINBridge
Path: /System/Library/Image Capture/Devices/TWAINBridge.app/Contents/MacOS/TWAINBridge
Version: ??? (???)
PID: 354
Thread: Unknown

Link (dyld) error:

dyld: /System/Library/Image Capture/Devices/TWAINBridge.app/Contents/MacOS/TWAINBridge truncated or malformed library: /System/Library/Frameworks/TWAIN.framework/Versions/A/TWAIN (file is empty)
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While troubleshooting this problem, I remembered that my Photoshop CS (version 8.0) also used to crash at start-up while "building the Twain menu items". Once I removed the Twain plug-in from the Photoshop library, it worked OK.

Do you have any suggestions how to fix this? Should I update or re-install the apparently corrupted Twain files and how can this be done?

many thanks in advance
-anni


iMac G5 Mac OS X (10.3.9)