This came back in the Essentials (2.8) .log file for a File-Share to Sharepoint 2019 OnPrem migration which simply got stuck until killed.

I cannot find any documentation for what it means

!MESSAGE An error occurred while automatically activating bundle…

In Essentials there is a list of invalid characters which can be replaced during a migration.

Up to version 2.5 it was possible to add additional characters to the list.

In version 2.7 this is still a documented feature in the accompanying user manual…