Damn. Tried disabling all protection on Kaspersky Total security. Errors happened too. Tried uninstalling Kaspersky. It started working again!
This has made me nuts. Im troubleshooting this since 10 hours ago.
Kaspersky is the problem.
For any still unknown reason (for me), kaspersky blocks connection between pc and duet wifi. Kaspersky was installed since a few months. Some update has made this mess.
Thank you all!

Tried connecting on Firefox running inside a linux machine on virtualbox. Not working too.

When opening page on android web browsers, no errors showing on paneldue. Even when reloading pages. DWC opens on android with newer version 2.x and 1.x (http://duetip/reprap.htm). No errors at all.

This is my M122 dump result:

M122
=== Diagnostics ===
RepRapFirmware for Duet 2 WiFi/Ethernet version 3.1.1 running on Duet WiFi 1.02 or later
Board ID: 08DGM-956GU-DJMSW-6J9FA-3SD6R-TUMMG
Used output buffers: 3 of 24 (15 max)
=== RTOS ===
Static ram: 27980
Dynamic ram: 94120 of which 44 recycled
Exception stack ram used: 264
Never used ram: 8664
Tasks: NETWORK(ready,636) HEAT(blocked,1224) MAIN(running,1880) IDLE(ready,80)
Owned mutexes: WiFi(NETWORK)
=== Platform ===
Last reset 00:00:57 ago, cause: software
Last software reset at 2020-05-23 15:19, reason: User, spinning module GCodes, available RAM 8672 bytes (slot 1)
Software reset code 0x0003 HFSR 0x00000000 CFSR 0x00000000 ICSR 0x0041f000 BFAR 0xe000ed38 SP 0xffffffff Task MAIN
Error status: 0
MCU temperature: min 28.9, current 29.7, max 29.8
Supply voltage: min 12.3, current 12.3, max 12.4, under voltage events: 0, over voltage events: 0, power good: yes
Driver 0: standstill, SG min/max not available
Driver 1: standstill, SG min/max not available
Driver 2: standstill, SG min/max not available
Driver 3: standstill, SG min/max not available
Driver 4: standstill, SG min/max not available
Date/time: 2020-05-23 15:20:12
Cache data hit count 97470607
Slowest loop: 4.22ms; fastest: 0.13ms
I2C nak errors 0, send timeouts 0, receive timeouts 0, finishTimeouts 0, resets 0
=== Storage ===
Free file entries: 9
SD card 0 detected, interface speed: 20.0MBytes/sec
SD card longest read time 2.3ms, write time 0.9ms, max retries 0
=== Move ===
Hiccups: 0(0), FreeDm: 169, MinFreeDm: 169, MaxWait: 0ms
Bed compensation in use: none, comp offset 0.000
=== MainDDARing ===
Scheduled moves: 0, completed moves: 0, StepErrors: 0, LaErrors: 0, Underruns: 0, 0 CDDA state: -1
=== AuxDDARing ===
Scheduled moves: 0, completed moves: 0, StepErrors: 0, LaErrors: 0, Underruns: 0, 0 CDDA state: -1
=== Heat ===
Bed heaters = 0 -1 -1 -1, chamberHeaters = -1 -1 -1 -1
=== GCodes ===
Segments left: 0
Movement lock held by null
HTTP is idle in state(s) 0
Telnet is idle in state(s) 0
File is idle in state(s) 0
USB is idle in state(s) 0
Aux is idle in state(s) 0
Trigger is idle in state(s) 0
Queue is idle in state(s) 0
Daemon is idle in state(s) 0
Autopause is idle in state(s) 0
Code queue is empty.
=== Network ===
Slowest loop: 199.67ms; fastest: 0.09ms
Responder states: HTTP(2) HTTP(0) HTTP(0) HTTP(0) FTP(0) Telnet(0), 0 sessions
HTTP sessions: 2 of 8

• WiFi -
  Network state is active
  WiFi module is connected to access point
  Failed messages: pending 0, notready 0, noresp 1
  WiFi firmware version 1.23
  WiFi MAC address 80:7d:3a:15:81:11
  WiFi Vcc 3.39, reset reason Unknown
  WiFi flash size 4194304, free heap 23408
  WiFi IP address 192.168.1.51
  WiFi signal strength -51dBm, reconnections 0, sleep mode modem
  Socket states: 0 0 0 0 0 0 0 0

@Phaedrux
I've tried that too.
Even replaced and reformatted the sd card. Using fresh new downloaded sd card contents.

Hi. Tried everything regarding browser version, cache, cookies. Even tried using portable browsers.

On android, every browser tested (Opera, firefox, chrome), everything is ok.
Using latest dwc version 3.1.1 all went ok on android.
Opening http://duetip/reprap.htm works even on my tv browser (which i dont know what it is). But keeps disconnected on pc browsers.

Re: Duet 2 WiFi - Web Control Connection Issues

I am having the following issue when connecting to duet wifi from pc chrome/firefox/opera browsers:

Blank screen on pc browser and "WiFi reported error: incomplete write" on paneldue.

On android chrome and opera browsers, everything is ok. Tried to rollback dwc versions and wifiserver versions. No luck.
When rolled back to dwc 1.22.6 the page opens but it cant connect.

F12 debug screen shows:
192.168.1.51/:79 GET about:blank net::ERR_UNKNOWN_URL_SCHEME
192.168.1.51/:1504 GET about:blank net::ERR_UNKNOWN_URL_SCHEME
GET http://192.168.1.51/js/dwc.js net::ERR_CONNECTION_RESET 200 (OK)