Here is the exception:

InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: Smith.DNN.Modules.BuyNow.MyCart.SaveToCartTable
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at Smith.DNN.Modules.BuyNow.MyCart.SaveToCartTable(DataTable myProductList) at Smith.DNN.Modules.ProductDetail.ProductDetails.AddToCart(Int32 ProductID, Decimal Quantity, Decimal price, Boolean isRequired, Int32 bundleID, Boolean inBundle, Decimal myWeight, Int32 catID) at Smith.DNN.Modules.ProductDetail.ProductDetails.ibAddtoCart_Click(Object sender, ImageClickEventArgs e) at System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) at System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) at System.Web.UI.WebControls.ImageButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---

Still occurs even after a clean reinstall and reload of the database.

Going back to 3.90. And another reload.

I just ran into the same problem...3.93, error adding product to cart from product details. Compared error message with what was posed and they match.

Added hot fix but unable to tell if it made a difference. Am now getting error below when going to the buynow page (which also has quick order entry / problem reported separately).

A critical error has occurred.
Request for the permission of type 'System.Data.SqlClient.SqlClientPermission, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.

I had reset IIS. Here's the entire event log.

error=Request+for+the+permission+of+type+%27System.Data.SqlClient.SqlClientPermission%2c+System.Data%2c+Version%3d2.0.0.0%2c+Culture%3dneutral%2c+PublicKeyToken%3db77a5c561934e089%27+failed.&content=0
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.12) Gecko/20101026 Firefox/3.6.12 ( .NET CLR 3.5.30729)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: f7a83d90-9aea-4ac5-91e7-5b8739ccac6f
InnerException: Request for the permission of type 'System.Data.SqlClient.SqlClientPermission, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Security.CodeAccessSecurityEngine.Check
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: Request for the permission of type 'System.Data.SqlClient.SqlClientPermission, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed. ---> System.Security.SecurityException: Request for the permission of type 'System.Data.SqlClient.SqlClientPermission, System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed. at System.Security.CodeAccessSecurityEngine.Check(Object demand, StackCrawlMark& stackMark, Boolean isPermSet) at System.Security.PermissionSet.Demand() at System.Data.Common.DbConnectionOptions.DemandPermission() at System.Data.SqlClient.SqlConnection.PermissionDemand() at System.Data.SqlClient.SqlConnectionFactory.PermissionDemand(DbConnection outerConnection) at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory) at System.Data.SqlClient.SqlConnection.Open() at Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.DiscoverSpParameterSet(SqlConnection connection, String spName, Boolean includeReturnValueParameter, Object[] parameterValues) at Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.GetSpParameterSetInternal(SqlConnection connection, String spName, Boolean includeReturnValueParameter) at Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.GetSpParameterSet(String connectionString, String spName, Boolean includeReturnValueParameter) at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(String connectionString, String spName, Object[] parameterValues) at Smith.DNN.Modules.BuyNow.SqlDataProvider.GetStoreSettings(Int32 PortalID) at Smith.DNN.Modules.BuyNow.SmithController.GetStoreSettings(Int32 PortalID) at Smith.DNN.Modules.MiniCart.MiniCart.Page_Load(Object sender, EventArgs e) at System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) at System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) at System.Web.UI.Control.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) The action that failed was: Demand The type of the first permission that failed was: System.Data.SqlClient.SqlClientPermission The first permission that failed was: The demand was for: The granted set of the failing assembly was: The assembly or AppDomain that failed was: App_SubCode_Smith.BuyNow, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null The method that caused the failure was: System.Data.IDataReader GetStoreSettings(Int32) The Zone of the assembly that failed was: Internet The Url of the assembly that failed was: file:///C:/inetpub/wwwroot/EVC/bin/App_SubCode_Smith.BuyNow.DLL --- End of inner exception stack trace ---
Source:

I've looked, but really don't know what to look for. My permissions should be good since they haven't changed any and I'm listed as the dbo. Beyond that I'm out of my element.