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...

SSRS. Must declare scalar variable @Parameter1

Well, one of the errors in Reporting Services 2005 says:

An error occurred during local processing

An error has occurred during report processing

Query execution failed for data set 'Name of the data set'

Must declare the scalar variable @Parameter1

This can be pretty annoying especially when you know you have the report parameter and it used to work. What happens is that, due to some black magic that I cannot explain, the data source is loosing the list of parameters. So the fix is to either add the parameters one by one or just delete and recreate the data source and everything will work.