Ok. Here's one that works for me, in GW3.
1. Open scheduling page for PGUM, filter to B789.
2. Assign MX to one of the unscheduled planes.
3. Attempt to assign MX to another unscheduled plane, but don't select a B check day.
4. Receive MsgBox() error message to pick a valid day.
5. Scheduling window drops without allowing for correction.
However, I can't seem to pin down exactly the steps that causes it - i.e. I reloaded to type up the above instructions, and that did not allow for replicating the problem... So, I'm thinking there's a variation on the inputs to that error check that cause it to escape the pop-over box, but I just can't figure out which it is...
Ninja edit - just happened again - error was when adding flights. MsgBox() said "select at least one flight to add" and then dropped me back to the page. Then, when I did it again, it kept the pop-over open after the MsgBox().