InvokeServerMethodAsync - threading problems?

I have a datagrid that contains some data.

In this grid, there is an expand button. When the user clicks this button a nested grid appears, with data that belongs to the specific row.

The data for the nested grid come from a InvokeServerMethodAsync call that is called the first time the user hits this expand button. This works fine.

Then I added a refresh functionality to the grid, that when used, refetches the data for the master grid and calls InvokeServerMethodAsync for the nested grids. This is where I get an error, which I think has something to do with the fact that I am calling the same InvokeServerMethodAsync many times within a short time frame.

But why is this a problem?

We are using Silverlight DevForce 6.0.9

Message: System.InvalidOperationException: An EntityManager can only execute on a single thread. This EntityManager is authorized to execute on the thread with id=’1’; the requested operation came from the thread with Id=‘53’. Consider calling the EntityManager’s asynchronous methods; they work safely on background threads managed by DevForce.  You may have to disable this cross-thread checking for specific reasons such as automated testing. Please review our documentation on multi-threading issues and the EntityManager.AuthorizedThreadId property.