FTP Problem

Job ID: 36999388

Budget: $10 – $30 USD

I have set up an FTP from where clients download files daily.
For years working fine, but suddenly today, I have a lot of random failures.
I have tried everything from upgrading my FileZilla server version etc.
The error it shows to clients is "Unable to connect to remote server".
But it's random, you press once to connect, it connects fine.
The connect button is pressed again, it connects well.
The third time the connect button is pressed and the error is thrown.
I've wasted all day trying to find the culprit, router, firewall, filezilla, configuration, etc.
I would look for an expert who can take me to the point of the situation and solve the problem right away.
Thank you.