Connection error during "Reading remote directory"

I am trying to set up a connection for employees in a medical setting. I have all the connection info and settings set:
The host name is correct.
The port is 990.
It's set to FTP and Explicit.
Username and password are correct and saved.
Here's the issue:
When I set up the site one machine, it connects with no problem.
I set up an identical site on a different machine, it gets to the "Reading remote directory..." and hangs up. It then pops up an error saying "Could not retrieve directory listing. Error listing directory '/'."

Things I've tried / tested:
2 machines. One connects, one doesn't.
Tried running WinSCP as admin thinking it was a permissions thing, it failed.
Compared all the advanced settings on both computers (even though none of them were changed during the setup) and both are identical.
Saw a forum saying they fixed it by forcing it to passive mode by unchecking and rechecking the passive mode box, so I tried that and it still failed.

So, I'm wondering if any of the people smarter than me have any insight? xD

Most likely a firewall does not let the FTP data connection through.
If you post session log files from both machines, we might investigate further.