Since i-MSCP 1.2.12, there is a new setup dialog asking you which port range you want use for the FTP server. If your're behind a NAT, you should in order:
Restrict this port range such as 33000 to 33500
nuxwin,
what is the expected port range for this dialog? my server uses a range of 200 somewhere in the 61000s, but the dialog did not accept that. it is possible to set passive ports in proftpd.conf afterwards.
best regards
v.