Software with backend SQL database throwing error

We have a backend POS software solution using SQL database as it's backend. When you open up the vendor, it shows an error screen "object reference not set instance of object", so I just press continue. It has a vendor catalog and you can choose to import items to the item database. And then it throws this error below. I'm not real familiar with SQL. Does this mean I have multiple problems or is it just be one problem.

String or binary data would be truncated.
The INSERT statement conflicted with the FOREIGN KEY constraint "FK_UPC_Item". The conflict occurred in database "PosBe", table "dbo.Item", column 'Item_ID'.
The INSERT statement conflicted with the FOREIGN KEY constraint "FK_VendorPartNo_Item". The conflict occurred in database "PosBe", table "dbo.Item", column 'Item_ID'.
The statement has been terminated.
The statement has been terminated.
The statement has been terminated.

Server stack trace:
at System.ServiceModel.Channels.ServiceChannel.ThrowIfFaultUnderstood(Message reply, MessageFault fault, String action, MessageVersion version, FaultConverter faultConverter)
at System.ServiceModel.Channels.ServiceChannel.HandleReply(ProxyOperationRuntime operation, ProxyRpc& rpc)
at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, TimeSpan timeout)
at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Exception rethrown at [0]:
at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
at PosBeData.PosBeService.GetAddCatalogItems(dtoV_VendorCatalog[] VendorCatalogItems, String Import_CD, Int32 Category_ID, Int32 Sub_Category_ID, Boolean CreateInventory, Boolean CreateCase, String Default_Mfg, UserAuth ua)
at PosBeData.V_VendorCatalogCollection.AddSelectedItems(String Import_CD, Int32 Category_ID, Int32 Sub_Category_ID, Boolean CreateInventory, Boolean CreateCase, String Default_Mfg)
at PosBe.frmVendorCatalog.AddItemsThread()

Note: When I first got this software it worked fine, a few weeks later it's not working. I tried to think what may have changed. I changed the computer name and installed some new SQL software, not sure if this could possibly be causing the problem. Also, the server needs to use Visual Studio 2010 Isolated, which when opens says 'Invalid license data. Reinstall is required.'

Any help would be greatly appreciated. Thanks