FTP problems

It could indeed be CSF. Depending on if you used the ./install.directadmin.sh or not.
At some places, like work or friends, depending on routers and firewalls, direct FTP access is not always possible.
In that case try PASV ftp. However, to be able to use PASV FTP, the ports which use it should be open in CSF. By default the port range 35000:35999 is used for FTP.
If you have that open in CSF (inclusive port 20 and 21) all incoming and outgoing on TCP, then CSF is not causing the problem.

I don't know where "somewhere else" is, but be aware of the fact that some company's do not allow their employees to use FTP traffic from their work place.

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