At the version 3.12 in the function "public void Save(string newText, string comment, bool isMinorEdit, bool reviewVersion)" HttpUtility.UrlEncode() was replaced by Uri.EscapeDataString().
Since Uri.EscapeDataString() is limited to 32766 characters on .NET 4 and 65500 characters on .NET 4.5, DotNetWikiBot raise an System.UriFormatException (Invalid URI: The Uri string is too long).
It's a trouble.
Thank you for reporting that.
Fixed in v3.13.