c# LinqToSql - TransactionIsInDoubt exception

Question

I have this simple webapi method the use linqtosql to execute some stored procedure:

[Authorize]
[Route("save")]
public IHttpActionResult api_Offert_SAVE(reqOffertSave req)
{

    try {

        using (var transaction = new TransactionScope())
        {
            var db2 = new OffertAppContext();

            db2.Offert_CLEAR(req.idOffert);

            db2.Offert_UPD_Header(req.idOffert, null, req.Title, req.Category, req.DatePickUp, req.DateDelivery, req.idDeliveryType, req.idPaymentType, req.PaymentDetail, req.ShippingDetail,req.idOffertSequel, req.Note);
            
            foreach (var r in req.Regions)
            {
                int? _idOffertRegion = null;
                db2.Offert_ADD_Region(req.idUserLastEdit, req.idOffert, r.Region,ref _idOffertRegion);

                var products = req.Products.Where(x => x.idOffertRegion == r.idOffertRegion).ToList();
                foreach (var p in products)
                {
                    int? x = null;
                    if(p.idProduct == 0)
                        db2.Offert_ADD_ProductDummy_FULL(req.idUserLastEdit, req.idOffert, _idOffertRegion, p.Product, p.idProductService, p.Qty, p.idUnitPrice, p.PriceCustomer, p.SeatsCustomer,p.Discount, ref x);                            
                    else
                        db2.Offert_ADD_Product_FULL(req.idUserLastEdit, req.idOffert, _idOffertRegion, p.idProduct, p.idProductService, p.Qty, p.PriceCustomer, p.SeatsCustomer,p.Discount, ref x);

                }

            }
            db2.Offert_CALCULATE_Totals(req.idOffert);
            transaction.Complete();
        }
            

        return Ok(1);
    }
    catch (Exception e)
    {
        return BadRequest(e.Message);
    }
}

Al was working fine till yesterday when I have updated the dbml file...

I wonder why I get transaction problem... The application is not in production and in this moment I'm the only who access the database...

In this method I only call stored procedures, why I get "transaction is in doubt" exception ?

MSDTC is enabled on my machine.

As suggested, I checked this question:

Reason for System.Transactions.TransactionInDoubtException

But I'm in a completely different and flat scerario than that...

NOTE:

After some checks I find out that if I remove this line:

db.Offert_CALCULATE_Totals(req.idOffert);

The error stops...

Thanks to support