add below tag in web config file in report manager folder.

<appSettings>
<add key="aspnet:MaxHttpCollectionKeys" value="100000" />
</appSettings>

Если не помогает - модифицируешь ReportServer web.config:

Add appSettings between /system.web and runtime:

</system.web>
<appSettings>
<add key="aspnet:IgnoreFormActionAttribute" value="true" />
<add key="aspnet:MaxHttpCollectionKeys" value="100000" />
</appSettings>
<runtime>