Hi, Jon Chen I want to help you with this problem.
As I understand, you have two users who are not able to login into Teams with their email addresses, right?
Please check the following.
1.) Do the users have the right permissions and is the team's license assigned (if they are members of the tenant)
2.) If the users are guest users, check your guest user settings in the MS Teams admin center.
3.) Are the users members already members of an MS Teams group (team)? Try to rejoin the user into the MS Teams team.
4.) When the problem only exists on the client (so in the web app it works), it has to be a problem with the client. I recommend uninstalling Teams from your device. Browse to the %appdata%\Microsoft folder on your computer and delete the Teams folder. Download and install Teams again.
If all of these are fine, try to delete and reinvite the users into the entra id tenant.
I know that the MS Teams product team reported they had a problem with such behavior recently.
The workaround was to use the classic Teams client instead of the new one.
If this solves your problem, I would recommend opening a ticket at Microsoft support.
I hope these approaches help you to solve the problem.
If the reply was helpful, please don’t forget to upvote or accept it as an answer, thank you!