In this case, the site will be turned off.

Trying to track back my brain. In some cases you may have to add CWD (Change Working Directory) to the list of commands available in line 16 of /etc/proftpd.d/flex-21.conf. As that file is generated every time you make changes to your flexshares, then you should really modify the generating program, /usr/clearos/apps/flexshare/libraries/Flexshare.php line 2761 so CWD is added every time flexshares are regenerated.

As I said, I don't know where the fault is between ProFTP and the Android app, but your problems do tend to point to client issues.

I have set up OpenVPN, as you suggested, and it is working, but correct me if I'm wrong. Every user, that connects through OpenVPN then gets my external IP address, right?

What do you mean that you hacked the ClearOS app? Proftpd app? And what is a CWD command?

As far as FTPS goes. If I log in with Total Commander in Windows, I don't have to specify the landing directory, it works without it, but if I want to log in with Android, then I have to specify the landing directory. And this is strange to me. Why in Windows it works, but in Android it doesn't work? Is this a "fault" on the Proftpd app (must something be enabled in config?), or because of some weird Android restriction?

It's also strange to me, that if I try to login with Pot Player on Windows or VLC Player on Android on port 2121 it connects, but on port 21 and port 990 it doesn't work...
Also on Pot Player it plays videos on port 2121, but on VLC Player, while it connects and you can browse through files, it doesn't play videos...

Yes, I'm trying to set remote access to videos and other files.
I have set up OpenVPN, as you suggested, and it is working, but correct me if I'm wrong. Every user, that connects through OpenVPN then gets my external IP address, right?
And another "problem". Can OpenVPN limit bandwidth like in proftpd, so that one or two users don't use the whole bandwidth?

The Application Filter is very different.

When you want to make an IP exception, is that a destination IP on the internet or a source IP on your LAN? In reality you don't use IP's at all, you need domains for the internet and devices (MAC addresses) for the LAN.

Have a look at the second answer here using a .htaccess file.

GM does not know the difference between http and https as it blocks at the DNS level.

The status where goes dead? What is your default policy and what policy group is your device in?

Is it running. Check from the webconfig for the SMTP server and IMAP/POP server. If one or the other is not running, check your logs.