OK, to close the loop on this:
The ultimate problem was the prof was not checking "Load This Tool In A New Tab" in Canvas. Canvas was loading Webwork in an iframe. This didn't *cause* the "unable to retrieve your permissions" error, but it did mean when students got the error and followed the instructions to click the BACK button, they would be unable to re-authenticate properly.
The prof changed their Canvas problem set links and students are now able to successfully complete their homework. The issue with needing to open in a new window was actually known and documented, but no one had connected it with this particular problem before.
As far as I can tell there's nothing different in the behavior of 2.18 vs. 2.16 here; it just seemed that way because of the timing of our upgrade.
Thanks for your patience.
The ultimate problem was the prof was not checking "Load This Tool In A New Tab" in Canvas. Canvas was loading Webwork in an iframe. This didn't *cause* the "unable to retrieve your permissions" error, but it did mean when students got the error and followed the instructions to click the BACK button, they would be unable to re-authenticate properly.
The prof changed their Canvas problem set links and students are now able to successfully complete their homework. The issue with needing to open in a new window was actually known and documented, but no one had connected it with this particular problem before.
As far as I can tell there's nothing different in the behavior of 2.18 vs. 2.16 here; it just seemed that way because of the timing of our upgrade.
Thanks for your patience.