VS2008 Cannot debug web site anymore

Question

Hi,

I can't debug my web site anymore in VS2008. I can set breakpoints and it stops at them. I can "move" the next instruction locator. After running string foo="foo"; I can give a new value to foo using the spy. But I can't rewrite this line as string foo="another value";

The debugger starts complaining about lack of source code or source being different of what was compiled. What should I start with ?

config : VS2008, vista, website in a classic (pipeline) pool in IIS with 1 worker thread