Same issue here while trying to upgrade Seblod from "3.28.1" to "4.3.1" after the migration from j3 to j4 (4.4.8):
This extension does not provide a checksum for validation of the integrity of the downloaded file.
0 Class "JCckDatabase" not found.
And also tried to install the latest Seblod (4.4.1) package but there is still a problem.