Lectora 19: 404 in Preview and Run modes!

In a course of mine, switching to either Preview or Run gives me HTML 404 ... in the development environment! Preview in Browser (F9) does the same, and if I publish the course to ReviewLink, an OnMouseClick "Go to next page" action is null-clicking the button does nothing



EDIT: it's ignoring Alt-F4. The Close button supplied by Windows did manage to close it.

EDIT FURTHER: closing all Lectora instances and restarting resolved the 404. Still weird.

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