Hox to fix a `proftpd-basic` installation deadlock on Debian with sysvinit?

apt install proftpd-basic

01

I have fixed it by executing service proftpd stop in another terminal:

02

The first terminal has finished the ProFTPD installation in this case:
03

The bug is described here: bugs.debian.org/cgi-bin/bugreport.cgi?bug=870624