Had the same error.
For me, it lookes like OMV did not enable the TLS module for ProFTPD by default. I think it does not for everyone.
In order to fix that, open a terminal and edit the file /etc/proftpd/modules.conf.
You can do that by typing, for example nano /etc/proftpd/modules.conf.
Then scroll down a little, you'll find these lines:
Remove the # from the second one, so that it looks like this:
Then restart ProFTPD (either by typing systemctl restart proftpd in the terminal, or by disabling and re-enabling from your OMV website and applying the changes each time).
Hope this will work for you too!
I use jonarin/glftpd in docker
It's a bit painful at the start regarding config, but GLFTPD still is the best FTP server out there today although it has been out there since ever