Interesting. I've seen the wifi firmware requiring a separate reflash after updating, but not seen the www folder missing.
Was it there originally?
How did you update the firmware and DWC to begin with?
What version were you coming from?
I would suggest downloading this zip file
https://github.com/Duet3D/RepRapFirmware/releases/download/3.1.1/Duet2and3Firmware-3.1.1.zip
And then uploading it as is to the /sys folder in DWC. That should prompt a firmware update as well as update any and all DWC files. That way you can be sure you have all the required files and verify that the firmware update process is working.
Use M122 to check the firmware version for the board (at the top) and wifi (at the bottom). And then check the DWC settings tab for the DWC version.
Firmware and DWC should be 3.1.1 and wifi should be 1.23.