Private app user login fails

I have a private app:

About 10% of users, who received a link to the app, have reported that they get this message:

All users who have reported the issue are in the user table.
All users who have reported the issue are able to log into the app if they try again- sometimes on the second try, sometimes on the third.

Is this an issue on Glide’s servers???
I cannot see how this can be an issue with the way that I’ve set up my app.
Can anyone help me understand what’s happening here and how I might try to resolve?
Glide support has been unhelpful, telling me that they cannot recreate the issue, so they cannot help.