PS CS4 crashes when trying to open PNG files

10-11  Source: Network gathering  Views:1 

I am using a freshly installed CS4 on a freshly installed Mac OS 10.6.3. No extra filters, nothing.
Now: whenever I export a PNG file from Illustrator via "Save for Web" and then try to open it in PS, the app crashes (endless beachball delight).
The same happened with a GIF file from AI. Only JPEG files work. Is this a recurrent problem and, if yes, why?
When I do the same at home with CS3, everything is fine.
Meanwhile, I get a spinning beachball in any CS4 app whenever trying
to open or save files, so I suspect a network issue (I'm on a large network).
To your question: I'm trying to open a file in PS which I created in AI. This is quite common practice. Open, paste, place or whatever. It's all the same result.
I already did the basic system maintenance. I don't think trashing prefs will be of any use as the same problem now also occurs in Fireworks and AI.
Update: I localized the problem to either the network or the external Firewire drive I use for Time Machine. It's from the infamous LaCie brand and has already caused trouble with the finder.
2nd update: When the network and the drive are disconnected, all open and save problems go away *but* the one with opening PNGs in PS when they were created in AI.
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