A few students cannot login - login process just loops

We’ve been using SketchUp for Schools for a few years with good success (thanks). This term we have 3 out of about 110 students who can’t login. Instead of logging in successfully, the login goes through the same loop as it does for students who have not Added SketchUp to the Allowed list of domains for pop-ups and re-directs. The pop-ups and redirects for the students who can’t get in are the same as for those who can. What else can be preventing the login?
The domain is nsd42.net, and it’s been whitelisted for a couple years now (I think).

Dan G
Elm Street Middle School
Nashua NH