Putty refuses to open serial ports and just dings

There's a windows 10 machine on which Putty refuses to open serial ports. There's no error message, just a "ding" then I click on Open. Any ideas on how to debug this?

I have tried rebooting and updating Putty already. The USB to UART dongles I'm trying to use are working fine when connected to a Linux PC and queried with screen /dev/ttyUSB0 115200. I've tried several such dongles, but the one I really want to use is a genuine FTDI FT2232H, which looks perfectly fine in device manager, with correct drivers installed.

OK, this one is rather stupid: I forgot to select the "Serial" radio-button on the Session tab and went straight to the Serial tab. Putty was still expecting an SSH connection and dinging because no IP address was given.