Make sure this mailbox is configured with Autodiscover, due to the basic authentication disabled in Office 365, there will exist issue when you try to configure mailbox in POP/IMAP.
Check whether those two mailboxes assigned with the same licenses from Microsoft admin center first. Mark sure all licenses are valid.
Then if this phenomenon occurs again, try to configure this mailbox in Outlook for PC client, check whether there exists with this mailbox.
You can also have a check about the service health, check whether there exists issue in your tenant.
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