The problem: Tinkercad Circuits sketch sharing links which include the term 'tenant=circuits' appear consistently to return a 404 error instead of accessing the
sketch. In particular, these are generated when creating a circuit and then immediately sharing it without ever exiting and reloading.
The first workaround appears to be to exit and reload the sketch at least once before generating the share link.
Alternatively, an existing link can be fixed by editing out the tenants=circuits clause, e.g. delete the text
I would consider this a system bug, as my expected behavior is that all generated sharing links should work, regardless of whether the sketch is new or not. On my first Fall 2020 assignment, two-thirds of submitted student links failed to work as students had created and submitted in one step.