SSRS - "attribute class has not been declared"

Hi,

I made an SSRS report, works well in Visual Studio. Only problem is, when I want to set it to a Output Menu Item in AX, I got the following: "xxx attribute class has not been declared" when compiling the menu item. I have the xxxDP and the xxxController classes working and compiling without any issue. Could you give me tips where to find the root of the issue?

Thanks in advance!