C# – The CLR has been unable to transition from COM context […] for 60 seconds

I am getting this error on code that used to work. I have not changed the code.

Here is the full error:

The CLR has been unable to transition from COM context 0x3322d98 to COM context 0x3322f08 for 60 seconds. The thread that owns the destination context/apartment is most likely either doing a non pumping wait or processing a very long running operation without pumping Windows messages. This situation generally has a negative performance impact and may even lead to the application becoming non responsive or memory usage accumulating continually over time. To avoid this problem, all single threaded apartment (STA) threads should use pumping wait primitives (such as CoWaitForMultipleHandles) and routinely pump messages during long running operations.

And here is the code that caused it:

var openFileDialog1 = new System.Windows.Forms.OpenFileDialog();
openFileDialog1.DefaultExt = "mdb";
openFileDialog1.Filter = "Management Database (manage.mdb)|manage.mdb";

//Stalls indefinitely on the following line, then gives the CLR error
//one minute later.  The dialog never opens.
if(openFileDialog1.ShowDialog() == DialogResult.OK)
{
    ....
}

Yes, I am sure the dialog is not open in the background, and no, I don't have any explicit COM code or unmanaged marshalling or multithreading.

I have no idea why the OpenFileDialog won't open – any ideas?