I am using Vista and sometimes I have to use ArchiCAD 9 on it, while ArchiCAD 9 is not supported under Vista, having been released much earlier.
I found that if I go to any other program, ArchiCAD 9 hangs and I have to force quit it.
However, recently I accidentally found the solution:
I go to any other application, select something and press CTRL+C to copy it to the Clipboard. ArchiCAD 9 miraculously comes back to life and continues to work fine. If it happens to hang again, I repeat the same fix and it is again in good working order.
Maybe it happens only on my machine, but let's hope this info is useful for others, too.
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