Hy, thanks for reporting. This issue is caused by the file browser window going behind the Export window. There are a few possible workarounds to this bug.
- Disable single window mode from preferences (this will actually show separate windows and you'd be able to refocus the file dialog if it accidentally hides behind).
- as it's a popup so theoretically (i haven't checked) pressing Escape key should hopefully allow you to interact with UI again.