FTP suddenly stops working after a KPN modem update or the switch to fibre

We have now received many reports from customers that FTP has suddenly stopped working. Timeouts occur.

This turns out to be caused by the modem update or by the switch to fibre. Make sure that ports 20 and 21 are open and, if you use passive FTP, that ports 35000 to 35999 are open as well.

Allowing everything in the KPN modem also solves it, but is not entirely secure.

With the settings shown in this list, active FTP will work. If you also want to use passive FTP, ports 35000 to 35999 must be opened.