I was member of a team in an external organization, when I used a free version of Teams. When I changed my office license from Family to business (with same email address) to get access to Teams for business, the guest login no longer works, neither with webversion or desktop version of Teams. Even though the extrenal organisaztion removed me from the team and invited me again. Microsoft support team (private) has tried to help for an hour, but can't fina a solution. anyone here to help?
Try reaching out to the external org. and have them remove your account completely from Azure AD, not just the team, so delete and also permanently remove from the Azure AD bin as well. After 24 hours they can invite you from their Azure AD again. When you have accepted that invitation you'll be added to their org. and will appear when inviting you from within the Team. It is possible to invite directly from the team after the removal, but when these kind of issues appear it's usually necessary to use the former approach.