By default Plesk -> ISP Manager migration has many problems. These steps will help you to process the normal migration.
1. When you are creating the backup file in Plesk use key -z to pack the archive without compression (by default Plesk uses compression which will unable it further extracting on ISP Manager server).
2. Change the archive extension to .tar
3. Move the archive to the ISP Manager server.
4. Parse it normally with Plesk import tool from ISP Manager panel functions.
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