But they can't even log in at this point. The error comes up after they enter their username and password. If I try to become them, I run into the same problem.

Now - something new I am researching. This was an import from Classic and I am noticing that some of the users in question have two accounts (one for them and one for their kids - SOP for us), but both with the same email address. Classic allowed this, but maybe Threads does not? Could this be causing the problem? IF so, haw can I rectify it?