Modifying the ReportManger web.config didn't work for me.

However, when I modified the the ReportServer web.config, the problem was solved.

Add appSettings between /system.web and runtime:

</system.web>

<appSettings>

...

I am also experiencing this. When I only select a few values in the parameters, it runs fine.

Also, my DEV environment is not experiencing this issue...

Any ideas?