Compiler runtime error after switching solution to VS 2010

Compiler runtime error after switching solution to VS 2010

we switched recently our asp.net 3.5 solution from VS 2008 to VS 2010
while still wanting it to be in .net 3.5 (as the deployment server runs on
3.5 only)
The libraries are ok, but the web site is causing us problems. The error
message we get on every aspx page is like this:
D:\WebSites\.....aspx: ASP.NET runtime error: Could not load type
'Microsoft.Build.Framework.ITaskItem2' from assembly
'Microsoft.Build.Framework, Version=3.5.0.0, Culture=neutral,
PublicKeyToken=b03f5f7f11d50a3a'.
I found out that the interface the compiler is looking for (ITaskItem2) is
only available in .net 4.0, but it keeps looking for it in 3.5 assembly
which is obviously wrong.
We were trying to locate where this discrepancy is originating, but with
no luck.
It hinders our work significantly, because the intellisense is not working
for asp components, also debugging of the web pages is not working.
But even when this error occurs in VS, the pages work ok when displaying
them in browser (using IIS), so this is VS-only problem.
If you have any idea, what could be wrong here, please let me know.
Thanks