I am writing an order entry app. The main data entry windows is an MDI child window, which contains a tabcontrol with 5 tabpages, each representing a step in the order entry process.
At the beginning of a cycle only a single tab page is shown; when a step is completed the next tab is shown. When an order is entered completely the tab pages are cleared and only the first tab is visible.

For data entry the enter key is used heavily to accept a selection. The focus shifts from one entry field to another and finally to the accept button.
During the first order everything works fine. When the focus is on a OK button and I hit the enter key, the application moves on.

Everything seems to work OK until I enter the second order. The enter key works OK on all entry fiels except for the OK buttons. These buttons respond to a mouseclick, but not to the enter key anymore.
A second observation is that it becomes impossible to close the MDI child window. I see that the form closing event is fired, but nothing happens after that.

This problem is making me crazy. Does anybody have a hint or a solution direction?

Thanks in advance