Bug#968967: Temporary fix by downgrading the package

[Thomas Walter]

I was having the same issue and had to downgrade the package on my
oldstable (9.13, amd64) systems.

grep "upgrade proftpd-basic" /var/log/dpkg.log
2020-08-23 19:47:01
upgrade proftpd-basic:amd64 1.3.5b-4+deb9u5 1.3.5e+r1.3.5b-4+deb9u1

To downgrade:
apt-get install proftpd-basic=1.3.5b-4+deb9u5

Regards,
Thomas