Can't add an external user to Teams, but CAN add their colleagues

Hello, I'm trying to add a few people who are external to my org to an MS Teams page. I have successfully been able to add 4 people from this other company but not one specific user. I keep getting the same error:

The only thing missing from this screen shot is their email address. There is no other content to the message.

All other posts on this topic lead to suggestions about administrators not providing rights to add external users, but this is clearly not the case as my company has a policy that allows external users and I've already added several other users from this same company to the same Teams page.

Thanks in advance