MessageDialog closing itself and returning cancel button?

In the course of updating an older project (written prior to the advent of the ThreadAccessingUI exception), I’ve run into a behavior I don’t understand.

In response to an error in a threaded operation, I need to display an error dialog. Previously, this was done directly from the thread using MessageDialog, and it worked fine. Now, I’ve implemented a short-period timer (the mode of which gets set to 1 in the thread), which then creates a MessageDialog object in its action event, and executes its showModal function. The message dialog does appear, but then disappears without any user intervention, returning the “_CancelButton.” If I change nothing, but replace the MessageDialog with a call to MsgBox, the modal dialog does not close itself. (I’ve also tried StandardAlertMBS, and it also leaves the dialog on screen until the user presses a button.)

This happens on both Xojo 2014r2.1 and 2014r3.1, running on Windows 7. (I haven’t yet tested on the Mac.) I am (of course) unable to duplicate it in a simple test project :stuck_out_tongue:

Can anyone suggest what might cause this behavior?