I have this query and I get the error in this function:
var accounts = from account in context.Accounts
from guranteer in account.Gurantors
I had the same error, when I tried to update some records within read loop.
I've tried the most voted answer MultipleActiveResultSets=true
and found, that it's just workaround to get the next error
New transaction is not allowed because there are other threads running in the session
The best approach, that will work for huge ResultSets is to use chunks and open separate context for each chunk as described in SqlException from Entity Framework - New transaction is not allowed because there are other threads running in the session