Unless, of course, you're actually working on the server, in which case you are perfectly correct in doing synchronous.
If this is the case, then you need to make sure that your EntityManager is not being used by multiple concurrent threads. If somehow the EntityManager was created on another thread, you can set the AuthorizedThreadId to 0 to turn off the check. However, only do this if you're absolutely sure you're not doing concurrent access.
Another suggestion is to make sure that you create an EntityManager for each server side request.