I found the service sheet and ran through the diagnostic cycles.
If I’m interpreting the sheet correctly, the only time the DETERGENT / RINSE AID DISPENSER is activated is in cycle 12. The solenoid clearly is energized and the detergent door does open. (I’m assuming this solenoid is also used to dispense the rinse aid at point in some rinse cycle.)
I’ll keep this info handy, however, I think the problem was a tray that shifted back when the basket was pushed in and it was just enough to block the door.
Dooh!
I would suggest checking the spring on the dispenser door, it appears that the door may open a crack but will not flip up to release the tablet from the dispenser and will not totally dissolve, if powder detergent it will usually be washed out of the cup.