Cannot connect through /dev/ttyACM0 (Ubuntu 18.04)
The board was bad. I just replaced it and it now works.

So, to answer my own question, an Arduino board can show up on lsusb and dmesg and yet be faulty. This is what the dmesg output is for a healthy board:

[10492.388028] usb 1-1.4: new full-speed USB device number 13 using ehci-pci
[10492.500520] usb 1-1.4: New USB device found, idVendor=2341, idProduct=0043
[10492.500526] usb 1-1.4: New USB device strings: Mfr=1, Product=2, SerialNumber=220
[10492.500529] usb 1-1.4: Manufacturer: Arduino (www.arduino.cc)
[10492.500533] usb 1-1.4: SerialNumber: 55736323739351010292
[10492.520354] cdc_acm 1-1.4:1.0: ttyACM0: USB ACM device
[10492.520832] usbcore: registered new interface driver cdc_acm
[10492.520835] cdc_acm: USB Abstract Control Model driver for USB modems and ISDN adapters


And here is the lsusb output:

Bus 001 Device 013: ID 2341:0043 Arduino SA Uno R3 (CDC ACM)


Judging on the difference with the previous outputs from both commands (when the faulty board was connected), it seems that my board had reverted to the basic Atmel USB interface and that the Arduino specific USB handshake was not working. Can anyone confirm ?