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...