Hi, I also upgraded my firmware, then ran into issues regarding the wifi module. So I DL the DuetWiFiServer-1.19.bin file and renamed it to DuetWiFiServer.bin
It wrote the firmware successfull but still I'm not able to start wifi module. Here is a log from pronterface. I have also tried restarting the duet wifi after firmware upgrade.
Connecting...
Printer is now online.
>>> M997 S1
SENDING:M997 S1
Trying to connect at 460800 baud:
success
Erasing 290848 bytes...
Uploading file...
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Upload successful
WiFi module started
Error: Failed to initialise WiFi module, code -10
[ERROR] Error: Failed to initialise WiFi module, code -10
>>> M552 S1
SENDING:M552 S1
WiFi module started
Error: Failed to initialise WiFi module, code -10
[ERROR] Error: Failed to initialise WiFi module, code -10
EDIT:
Hm, seems like it didnt upgrade the wifi firmware, shouldn't it be 1.19?
M115
SENDING:M115
FIRMWARE_NAME: RepRapFirmware for Duet WiFi FIRMWARE_VERSION: 1.19 ELECTRONICS: Duet WiFi 1.0 FIRMWARE_DATE: 2017-08-14
Running the firmware upgrade routine again won't change from 1.0 to 1.19 wifi firmware.
EDIT! For anyone else just seeing this post, read this page: https://duet3d.com/wiki/Upgrading_to_DuetWiFiFirmware_1.19
Solved all my issues! 
Do EXACTLY as it says.