The issue appears when a newly installed Migration Manager for Exchange server picks up the wrong license information from a previous Migration Manager server.
A new Migration Manager for Exchange server is installed and a new license file (eg. with 1,400 mailbox) is loaded. However this server shows the old license information (eg. 1,900 mailbox) from the previous migration (few years back) in the same source organization. No mailbox migration can be done.
The old license counters were still stored on the old Exchange servers that happened to participate in the earlier migration project. Once new Mail Source Agent /Calendar Source Agent were installed they are still picking old values.
To see how Migration Manager for Exchange licenses are counted, please refer to Solution 44530 - How does Quest Migration Manager for Exchange count licenses? - https://support.quest.com/SUPPORT/index?page=solution&id=SOL44530
Quest Migration Manager for Exchange keeps all mailbox license counters on the source Exchange server and they are not related to the instance of project so all counters will be added to each other. In such scenario, additional licenses should be obtained from the Quest Account Manager.