SSRS 2008 incorrect number of TablixCornerCell

I got a "nice" error today , while working on a report converted from SSRS 2005 to SSRS 2008.

Error 1 [rsWrongNumberOfTablixCornerCells] The tablix ‘matrix4’ has an incorrect number of TablixCornerCell. The number of TablixCornerCell within TablixCornerRow at index: 0 must equal the number of levels in the TablixRowHierarchy containing TablixHeaders.

Update , 2012-02-10: I fixed the problem by simply reading carefully the error message. All I did was adding an empty <TablixCornerCell></TablixCornerCell> into the <TablixCornerRow>

Sometimes VS 2008 cannot access DataSource file

Today, for the second time since we started using SSRS 2008 I had the following stupid error:
Running a dataset works from VS designed, but when I try to run the report (again, from the VS2008 designer) I get errors (DS couldn't be run)

After little investigation I realize that the problem is in fact that the report cannot be built. I had VS2008 access to RDS file denied. this is not a read-only file, is checked-out, so VS should be able to access the file and change it.
However, VS keeps telling that access to {MyReportsFolder}\bin\Debug\{MyDataSource.rds} is denied
The only way I found to fix this was to delete the folder {MyReportsFolder}\bin and rebuild.

Weird...