Last week I bumped upon this error while I was trying to do an asynchrounous WCF service call from my "MS Project 2007 Web Access"-custom user control.
Turns out that you cannot set the Async="true" atttibute in the page directive af an aspx page in the Project Web Access.
The reason for this? Very simple ... the that SharePoint’s SPPageParserFilter class blocks some page directive attributes. When you use other attrubutes than the following list, you will get the error "The 'xxx' attribute on the page directive is not allowed in this page."
To take care of this issue we chose to have a webservice call which will handle the Ascynhrounous WCF call, and from our web user control we called this webservice method sycnhrounous.... Maybe this is not the best solution, but we really had an issue, so a quick fix was necessary here...