I discovered by now that the problem with Tr does not affect all computers. I tried it on a different PC (32-bit instead of 64, but this can't be the reason, can it?) and it did work. Unfortunately the Help Browser does not work on that macheine: Clicking on an entry crashes Mathematica.
So the question is why does it work on some PCs and not on others? It looks more like a software bug than a hardware problem...
If anyone had the same problem and knows how to circumvent it, I would appreciate any tips.
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