Compare page versions genereates an error when comparing with markup or side by side.
Error displayed is: (System.Web.UI.HtmlControls.HtmlGenericControl) is not compatible with the type of control (System.Web.UI.HtmlControls.HtmlIframe).
My investigation shows that Microsoft replaced HtmlGenericControl with HtmlIframe, this should then need the source to be recompiled. https://connect.microsoft.com/VisualStudio/feedback/details/736011/iframe-parser-error
We are running EPi Server 6 R2 (6.1.379.502)
EPiServer 6 R2 does not officially support running .NET 4.5. I'm not sure if this change happened beween 3.5 and 4.0 or 4.0 to 4.5 but you need to apply the following changes to be able to run .NET 4.0 on a EPiServer CMS 6 R2 site:
Okey. the issue appears only on 4.5, on 4.0 it works fine.