I have spotted a strange problem which I have managed to get around but thought it worth sharing:
I have a report whose datasource was an IList of a custom object. The custom object's class has an int property; 'PropertyName' in this example. On my report I have a table of data and two charts, all bound to the same data source. The table rendered without any issues, however the chart came up with the error mentioned in the title and would not render. This report had worked initially so it surprised me to learn that a very minor change had caused this error.... if I change the datasource from an IList to a List the error disappears.
Hopefully this will save somebody the pain gone through to rectify this, and perhaps this issue could be looked into and maybe addressed.