At least 30-40% of my students per class are still getting this error on their Chromebooks. Today, though, a student thought to login to their create.arduino.cc account on one of our Macbook Pros and, after I installed the plugin, things started working smoothly. I tried this with several other students who were having the same error on their Chromebooks and it worked for all of them.
So it does seem to be a Chromebook-specific issue. Again, we are using Dell Chromebook 11 generation 2 Chromebooks.