I am getting this error also and my web.config has the suggested solution already. I saw this on another google post regarding DNN that is identical.
I have a datalist on a ascx page with two hyperlinks in it. The first hyperlink displays an image, the second displays text beneath the first one.
<asp:HyperLink id="dl1ProjectURL" runat="server" ImageUrl='<%</SPAN><SPAN style="COLOR: blue">#</SPAN> DataBinder.Eval(Container.DataItem, "ThumbnailImage") <SPAN style="BACKGROUND: yellow">%>' NavigateURL='<%</SPAN><SPAN style="COLOR: blue">#</SPAN> DataBinder.Eval(Container.DataItem, "ProjectURL") <SPAN style="BACKGROUND: yellow">%>' Target="_blank">asp:HyperLink>
<br /><asp:HyperLink id="dl1ProjectURL2" runat="server" Text='<%</SPAN><SPAN style="COLOR: blue">#</SPAN> DataBinder.Eval(Container.DataItem, "ProjectHome") <SPAN style="BACKGROUND: yellow">%>' NavigateURL='<%</SPAN><SPAN style="COLOR: blue">#</SPAN> DataBinder.Eval(Container.DataItem, "ProjectURL") <SPAN style="BACKGROUND: yellow">%>' Target="_blank">asp:HyperLink>
when I remove the hyperlinks When I navigate to an edit control from this ascx, and press the return button on that form which only has this;
I get the error above when it returns to the form with the hyperlinks in the datalist.
When I remove the hyperlinks the error does not appear.
The suggestion in the other google link I found was that the person replaced the hyperlink he used with an image button and the error went away. I will try this next and suspect it will work.
Now I'm using DNN 5.51 and this is occurring. I'm using the same code and techniques I have used in all my DNN moduels (UCanUse modules) and never had this issue before.
Hope this helps point to some type of solution.