Some parameter names not allowed in SSRS 2008 R2

I was suprized to realize that some previous well functioning SSRS reports failed when 'previewed' .
The error sound like : "An error occurred during local report processing. An error has occurred during report processing.Query execution failed for dataset '****'. Parser: The syntax for 'Date' is incorrect.

But the report worked once and as far as I could see nothing has changed behind time. So how can it be ?
It took me a while to realize, that all of a sudden it does not like the parameter name 'Date' , because the syntax around it has never changed. It looked like parameter names like @Date and @Application were not allowed any more. They were treated as reserved keywords.

So the solution was just dummy replacing them - by @Date1 , @Application1 or the like  - in query , parameter list and all the way around where they showed up.
Then I could preview the reports like I did it once.
If you also experienced this or have an idea of the root cause of this issue, please leave me a note.