You could just open the learning path and you will see all the modules see, which one is not at 100 % that one is the one your missing. If everything is on 100 % it could be that is just an error on Mendix side and you could just wait till it fix itself or push a feedback through.
Hope this will help your problem!
Yes I know which module is not complete but I do not know where IN THE MODULE the incompletion is. Any clues to how to determine this?
If you open the module wich is incomplete you can see on the left side the steps you have completed marked with a green check mark. If all is marked green then it is like Viet sayed an error on Mendix's side.