OK.
I have troubleshooted a bit. I'm using one of my first "real" keyboards here, at my parents' - a Logitech G15 from university. HC4's controller seems to just not like this keyboard.
I tried with the Logitech G15 and saw the scary
usb usb1-port2: connect-debounce failed
followed by nothing.
To test, I inserted a Razer Deathadder v3, and really nothing (lights didn't turn on).
I then started switching kernels, down to one I half remembered working - 6.6.63. With the keyboard, I now see a constant loop of
[ 226.077861] usb 1-2: Manufacturer: Logitech
[ 226.297560] hub 1-2:1.0: USB hub found
[ 226.297854] hub 1-2:1.0: 4 ports detected
[ 226.583532] usb 1-2.1: new low-speed USB device number 122 using xhci-hcd
[ 226.583808] usb 1-2: USB disconnect, device number 121
[ 226.584598] usb 1-2-port1: attempt power cycle
[ 227.207504] usb 1-2: new full-speed USB device number 126 using xhci-hcd
[ 227.357792] usb 1-2: New USB device found, idVendor=046d, idProduct=c223, bcdDevice= 1.03
[ 227.357813] usb 1-2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 227.357819] usb 1-2: Product: Logitech G15 Keyboard
[ 227.357823] usb 1-2: Manufacturer: Logitech
[ 227.577491] hub 1-2:1.0: USB hub found
[ 227.577766] hub 1-2:1.0: 4 ports detected
[ 227.863426] usb 1-2.1: new low-speed USB device number 127 using xhci-hcd
[ 227.863627] usb 1-2: USB disconnect, device number 126
[ 227.864513] usb 1-2-port1: attempt power cycle
[ 228.487407] usb 1-2: new full-speed USB device number 5 using xhci-hcd
[ 228.637379] usb 1-2: New USB device found, idVendor=046d, idProduct=c223, bcdDevice= 1.03
[ 228.637399] usb 1-2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 228.637405] usb 1-2: Product: Logitech G15 Keyboard
[ 228.637409] usb 1-2: Manufacturer: Logitech
[ 228.857489] hub 1-2:1.0: USB hub found
[ 228.857767] hub 1-2:1.0: 4 ports detected
[ 229.143365] usb 1-2.1: new low-speed USB device number 6 using xhci-hcd
[ 229.143566] usb 1-2: USB disconnect, device number 5
[ 229.144516] usb 1-2-port1: attempt power cycle
[ 229.767339] usb 1-2: new full-speed USB device number 10 using xhci-hcd
[ 229.917390] usb 1-2: New USB device found, idVendor=046d, idProduct=c223, bcdDevice= 1.03
[ 229.917415] usb 1-2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 229.917426] usb 1-2: Product: Logitech G15 Keyboard
[ 229.917434] usb 1-2: Manufacturer: Logitech
[ 230.137643] hub 1-2:1.0: USB hub found
also some
84.508853] usb 1-2.1: new low-speed USB device number 71 using xhci-hcd
[ 84.509553] usb 1-2-port1: cannot reset (err = -71)
[ 84.509688] usb 1-2-port1: cannot reset (err = -71)
[ 84.509813] usb 1-2-port1: cannot reset (err = -71)
[ 84.509950] usb 1-2-port1: cannot reset (err = -71)
[ 84.510080] usb 1-2-port1: cannot reset (err = -71)
[ 84.510087] usb 1-2-port1: Cannot enable. Maybe the USB cable is bad?
[ 84.510396] usb 1-2-port1: cannot disable (err = -71)
[ 84.510564] usb 1-2: USB disconnect, device number 70
[ 84.510679] usb 1-2-port1: attempt power cycle
[ 85.132133] usb 1-2: new full-speed USB device number 75 using xhci-hcd
[ 85.282023] usb 1-2: New USB device found, idVendor=046d, idProduct=c223, bcdDevice= 1.03
[ 85.282044] usb 1-2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 85.282050] usb 1-2: Product: Logitech G15 Keyboard
[ 85.282054] usb 1-2: Manufacturer: Logitech
[ 85.497699] hub 1-2:1.0: USB hub found
But it keeps repeating, so I also inserted the Deathadder, and lo and behold
[ 1705.912365] input: Razer Razer DeathAdder Essential as /devices/platform/soc/ffe09000.usb/ff500000.usb/xhci-hcd.1.auto/usb1/1-2/1-2:1.0/0003:1532:006E.0001/input/input1
[ 1705.913173] hid-generic 0003:1532:006E.0001: input,hidraw0: USB HID v1.11 Mouse [Razer Razer DeathAdder Essential] on usb-xhci-hcd.1.auto-2/input0
[ 1705.915894] input: Razer Razer DeathAdder Essential Keyboard as /devices/platform/soc/ffe09000.usb/ff500000.usb/xhci-hcd.1.auto/usb1/1-2/1-2:1.1/0003:1532:006E.0002/input/input2
[ 1705.973005] input: Razer Razer DeathAdder Essential as /devices/platform/soc/ffe09000.usb/ff500000.usb/xhci-hcd.1.auto/usb1/1-2/1-2:1.1/0003:1532:006E.0002/input/input3
[ 1705.973333] hid-generic 0003:1532:006E.0002: input,hidraw1: USB HID v1.11 Keyboard [Razer Razer DeathAdder Essential] on usb-xhci-hcd.1.auto-2/input1
[ 1705.974735] input: Razer Razer DeathAdder Essential as /devices/platform/soc/ffe09000.usb/ff500000.usb/xhci-hcd.1.auto/usb1/1-2/1-2:1.2/0003:1532:006E.0003/input/input4
[ 1706.081010] hid-generic 0003:1532:006E.0003: input,hidraw2: USB HID v1.11 Keyboard [Razer Razer DeathAdder Essential] on usb-xhci-hcd.1.auto-2/input2
[ 1990.380255] usb 1-2: USB disconnect, device number 63
So I suspect that it's just the HC4 disliking the old Logitech G15, if not even the G15 dying on me...