I purchased 3 After Effects projects from a third party website http://www.pond5.com/ and I downloaded the CS5 version on 2 of them and like CS4 on one and only the CS4 version works. Yes I tried searching their website for help but there is none. Just wondering if there is any other reason this might be rejected the error actually says something about attempting to open a project created with 10.0 and cannot be used with this version. I did an update as well no luck.
Make sure to apply all the pertinent updates. Also perhaps those CS5 files were saved down from CS5.5 and contain garbage. Likewise there may be third-party plug-ins involved. Same for fonts or specific cross-platform issues. So many things that could be wrong here. Without knowing what the projects contain, impossible to diagnose. If everything fails, you can always download the CS5.5/ CS6 trial and open it there...
Duh, I just remembered I bought CS5 but had to do CS4 for After Effects because my computer is not 64bit.