We encountered this error recently when we upgraded from CRM 2013 to CRM 2015. We had our plugin where we used to execute a custom SSRS report and get the byte content of the report and create an attachment for an entity. For this we had a reference to the Microsoft.ReportView.WebForms dll v 10.0
The code used to work perfectly in CRM 2013. However when we upgraded to CRM 2015, we started getting the above error. The reason for this is because CRM 2013 utilizes Microsoft.Crm.Application.Pages.dll v6.0 which is compatible with Microsoft.ReportViewer.WebForms. dll v10.0. However CRM 2015 used the Microsoft.Crm.Application.Pages.dll v7.0 which goes with Microsoft.ReportViewer.WebForms. dll v11.0.
Updating our project to refer to Microsoft.ReportViewer.WebForms. dll v11.0 resolved this issue.
Hope this helps!