Migration of a single subscription from Plesk 11 to Plesk 12 server fails with the following error:
PHP Fatal error: Call to a member function capability() on a non-object in /usr/local/psa/admin/plib/backup/Conflicts/RequiredSettingsResolver.php on line 47
Also no IP mapping asked in pre-migration steps.
The issue is considered to be a software issue #PPPM-1857 and was fixed in Plesk 12.
Please consider to upgrade panel to latest version in order to fix this issue.
If upgrade is not acceptable you could use following workaround:
Add 'A' record for 'mail.domain.tld' in source domain zone manually and restart migration.