Try the following code instead.

this.SaveOptions.TransactionSettings = new TransactionSettings(IsolationLevel.ReadCommitted, TimeSpan.FromSeconds(30), true);
var trxOptions = this.SaveOptions.TransactionSettings.ToTransactionOptions();

var scope = new TransactionScope(TransactionScopeOption.RequiresNew, trxOptions);
using (scope) 
{
..........Your code here............
}

1. Notice that I use a different TransactionScope(TransactionScopeOption, TransactionOptions) constructor here. There appears to be a bug in .NET when using the TransactionScope(TransactionScopeOption) constructor.

2. If you want to modify the TransactionSettings on the server, use this.SaveOptions.TransactionSettings instead. By the time the save gets to the server, it'll grab the TransactionSettings from SaveOptions and so this is the instance that you want to modify.

I have several tables; some are connected, some are not. During save, when I save a record in table A, it should create a record in table B. Table B has ID field that should be retrieved from table A's ID. However, there's no association between table A & B. I can't add association because ID field in table A is also used by other tables as foreign key. 

During Save process, the table A's ID saved in table B are not fixed up, because there's no association.

To solve this issue, I tried to use EntityServerSaveInterceptor to intercept the save process. Here's what I'm trying to do in the interceptor.

base.ExecuteSave();

//get ID value in table A

//update the ID field in table B

However, I get error: 

"The transaction specified for TransactionScope has a different IsolationLevel than the value requested for the scope."

I'm not sure whether this is allowed. If not, could anyone suggest a solution ? Thanks a lot !