Since updating to 1.20 release I'd started getting these. I did not get them running the earlier 1.20 betas (going by my downloads I used beta 1, 6 &10) I went to DuetWiFiServer 1.20+1 and 1.20.1RC and it actually seemed more frequent.
I'm now back to 1.20 for firmware, web, and wifi versions, and they're still happening every few minutes. It's the same error others are reporting in DWC: "The last HTTP request has timed out. Please make sure the connection between your device and the board is not interrupted."
Tomorrow I'll connect via usb and use M111 S1 P14 to grab some logs.