Hi Robert,
Based on the error message “the license for the mailbox has expired”, this issue could be related to the license. Therefore, I suggest you check the status of your licenses: please sign in to the portal as a global administrator, and then click licensing on the page, locate the licenses tab. You can view the status of your licenses here.
Are there any expired licenses? If so, you can renew the expired licenses to resolve this issue. If all the licenses are valid, may I collect some more information to help troubleshoot this issue?
1. Which Office 365 are you using, Enterprise plan or the standalone Exchange Online plan?
2. For the problematic users, can they access other online services? For example, can they access your SharePoint team site? Can they sign in to Lync?
3. About these affected users, please check if you can see their mailboxes in Exchange Admin Center.
Are their mailboxes listed here?
Thanks,
Young Yang