The problem seems to be related to missing or changed resource file. Check out whether you have the following declaration under ~/Sitefinity/AccessDenied.aspx
|<font style="color:Red;font:normal 18px Arial, Verdana, Sans-serif"><asp:Literal runat="server" Text="<%$Resources:NoPermission %>"></asp:Literal></font>
and the related resource file under App_LocalResources
Value: "Oops... you have no permissions to access this page!"
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