Hi Igor, support has fixed the issue, see
The issue was cause Bug ID #PPPM-1948 (Unable to change limits in Plesk GUI for Power User mode).
On the source server it was a subscription winjob.com.br with mailbox limit=12.
You migrated this subscription to Plesk 12 and limit=12 was migrated too, but since you are using Web Admin Edition License it's not possible to change limit using Plesk GUI(Bug ID #PPPM-1948).
But it possible to change mailbox limit using command line:
~]# /usr/local/psa/bin/subscription_settings --update domain.tld -max_box -1
This command will set unlimited mailboxes for domain.tld.
> I am afraid that this will happen again
Yes it will happen again. There are two way to avoid the issue:
1. Change limits for domain to unlimited on the source server and after that migrate subscription or downgrade license.
2. Use command line utility to change limits(see above) for more information you can use help :
~]# /usr/local/psa/bin/subscription_settings --help
Also you can check guide:
http://download1.parallels.com/Plesk/PP12/12.0/Doc/en-US/online/plesk-unix-cli/37982.htm