I am experencing the same error opening CS4 projects in CS5.
I am working on a 2x3.2 Quad Core Intel Xeon. 16 GB RAM memory.
I hope Adobe find a solution soon as possible.
I experienced the exact same issue. I had received some client files in cs5 that needed editing....i was still running cs4.
For some reason After Effects cs5 installed on my mac running osX 10.5.4. It requires osX 10.5.8. Adobe, why didn't this give me a warning?
Now, I don't know if it was the os update, or during that process it reset something critical......but after the operating system update the error no longer appeared.
Hope this helps.
Strange, I just experienced the same thing — but it seemed to clear as soon as I rebooted my machine.