Asp.net add multiply rows to CRM 2011 (2+ clients/User) - asp.net

Question

I've extended the customer portal from MS for 2011 with functionality to add an order.
Now i came across a problem when 2 or more user try to create an order at the same time.
first I created the SalesOrder with:
order = new SalesOrder();
order.SalesOrderId = OrderID;
order.Name = string.Format("Order {0}", DateTime.Now);
order.CustomerId = parentCustomer;
order.PriceLevelId = priceLevel.ToEntityReference();
OrderID = XrmContext.Create(order);
and after this i added products(SalesOrderDetail) with a foreach:
foreach (var product in OrdredProducts)
{
productToAdd = new SalesOrderDetail
{
SalesOrderId = new CrmEntityReference(SalesOrder.EntityLogicalName,
OrderID),
ProductId = new EntityReference(Product.EntityLogicalName,
product.ProductID),
Quantity = product.Quantity
};
XrmContext.Create(productToAdd);
}
this works fine when just one user orders something. But if 2+ users I get error for both user.
so I changed the code to this:
order = new SalesOrder();
OrderID = Guid.NewGuid();
order.SalesOrderId = OrderID;
order.Name = string.Format("Order {0}", DateTime.Now); //:d
order.CustomerId = parentCustomer;
order.PriceLevelId = priceLevel.ToEntityReference();
XrmContext.AddObject(order);
foreach (var product in OrdredProducts)
{
productToAdd = new SalesOrderDetail
{
SalesOrderId = new CrmEntityReference(SalesOrder.EntityLogicalName,
OrderID),
ProductId = new EntityReference(Product.EntityLogicalName,
product.ProductID),
Quantity = product.Quantity
};
XrmContext.AddObject(productToAdd);
}
XrmContext.SaveChanges();
Now if 2+ Users orders something, just one User fails not both like the first try.
the Errors I've got as far(every time an other one):
ValidateOpen - Encountered disposed CrmDbConnection when it should not be disposed
SQL timeout expired.
An unexpected error occurred.
Error retrieving next number (currentordernumber) for organization {guid): return value is empty
System.InvalidOperationException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #F60BF6A2
does anyone have some suggestion where the problem could be? is it the SQL Server/CRM Server/IIS/my code or something different?
Additional Information
SQL Server and CRM Server are not on same machine
I get the same error when i test in VS2012 server(back-end is the same SQL/CRM server)
i get the same error when one user runs over IIS and one over VS2012(debugg)
edit
XrmContext is a XrmServiceContext type and this is derived from CrmOrganizationServiceContext and this one is generated with the crmsvcutil

Answer 1

if someone gets the same problem i solved it following:
first i created the SalesOrder and added it to the CRM:
public void AddToCrm(Entity entity)
{
XrmContext.AddObject(entity);
}
then I created all Orders(SalesOrderDetail) and added them to the CRM(same function as above).
And finally i saved it:
public void SaveToCrm()
{
XrmContext.SaveChanges();
}
this behaviour is like a transaction in the database.
so all other Order have to wait till this one is done.