Migration from Plesk 12.5 does not completely migrate subdomains

I wanted to migrate a domain from the old server managed by Plesk 12.5 to the new server managed by Virtualmin 5.02.gpl

First, I did a backup of the domain using Plesk backup manager and then used Add Server --> Migrate Virtual Server (Virtualmin). The progress messages of the task are looking fine but for the 2 subdomains the html/php code was not uploaded to the new server. The code of the domain was installed correctly.
And the 3 database of the domain were created correctly as well.

Any hints how to solve the issue ?

FDM

==== Messages of migration task ====

Validating migration file …
… done
Starting migration of example.com from uploaded file …
Finding contents files …
… done
Checking for Plesk features …
… found Home directory, Administration user, Webmin login, Mail for domain, MySQL database.

Checking for clashes and dependencies …
… all OK

Creating initial virtual server example.com
Creating administration group fdmpoli …
… done
Creating administration user fdmpoli …
… done

Creating aliases for administration user …
… done

Adding administration user to groups …
… done

Creating home directory …
… done

Creating mailbox for administration user …
… done

Adding to email domains list …
… done

Creating MySQL login …
… done

Creating Webmin user …
… done

Re-loading Webmin …
… done

Saving server details …
… done

… done
Copying web pages …
… done

Copying CGI scripts …
… done

Re-creating mail users …
… done (migrated 2 users)

Re-creating mail aliases …
… done (migrated 1 aliases)

Migrating MySQL databases …
Creating MySQL database revive_adserver_3 …
… done
Creating MySQL database example-test …
… done
Creating MySQL database example …
… done
… done (migrated 3 databases, and created 3 users)
Re-creating protected directories …
… done (migrated 0)

Creating sub-domain adserver.example.com
Creating home directory …
… done
Adding to email domains list …
… done

Adding default mail aliases …
… done

Saving server details …
… done

Updating Webmin user …
… done

Re-loading Webmin …
… done

… done
Re-creating sub-domain users …
… created 0

Creating sub-domain testsite.example.com
Creating home directory …
… done
Adding to email domains list …
… done

Adding default mail aliases …
… done

Saving server details …
… done

Updating Webmin user …
… done

Re-loading Webmin …
… done

… done
Re-creating sub-domain users …
… created 0

Successfully migrated 3 servers. Click here to manage the new server.