Just updated my D-Bot and all good so far. No issues after update with warm and cold restarts and a successful small print.
Edit: Running a Duet Wifi

After seeing your reply I decided to reload the wifi firmware only and that seems to have done the trick. Tried a few restarts and all good. Thanks!

I am seeing the same after updating to 1.21RC1 from 1.18 last night. Sometimes it takes a few retries before it connects.
[c]
Connecting…
serial: M105
Printer is now online.
serial: M105
serial: M105

M122
SENDING:M122
serial: M122
=== Diagnostics ===
Used output buffers: 1 of 32 (1 max)
=== Platform ===
RepRapFirmware for Duet WiFi version 1.21RC1 running on Duet WiFi 1.0 or 1.01
Board ID: 08DAM-999TL-MQ4S8-6J1FJ-3SJ6L-16BHW
Static ram used: 15448
Dynamic ram used: 99736
Recycled dyWiFi:
namic ram: 3600
Stack ram used: 3568 current, 4520 maximuWiFi: xmit: 0
m
Never used ram: 7768
Last reset 00:13:04 ago, cause: software
Last software reset at 2018-02-12 00:16, reason: User, spinning module GCodes, available RAM 7808 bytes (slot 1)
Software reset code 0xWiFi: recv: 0
0003 HFSR 0x00000000, CFSR 0x00000000, ICSR 0x0441f000, BFAR 0xe000ed38, SP 0xffffffff
Error WiFi: fw: 0
[ERROR] Error WiFi: fw: 0

status: 0
Free file entries: 10
SD card 0 detected, interface speed: 20.0MBytes/sec
SD card longest block write time: 0.0ms
MCU temperature: min 31.9, current 32.4WiFi: drop: 0
, max 33.1
Supply voltage: min 13.8, current 14.0, max 14.1, under voltage events: 0, over voltage events: 0
Driver 0: standstill, WiFi: chkerr: 0
SG min/max not available
Driver 1: standstill, SG min/max not available
Driver 2: standstill, SG min/max not available
DriverWiFi: lenerr: 0
3: standstill, SG min/max not available
Driver 4: standstill, SG min/max not available
Date/time: 1970-01-01 00:00:00
Slowest main loop (seconds): 0.001301; fasteWiFi: memerr: 0
st: 0.000042
=== Move ===
MaxReps: 0, StepErrors: 0, LaErrors: 0, FreeDm: 240, MinFreeDm 240,WiFi: rterr: 0
MaxWait: 0ms, Underruns: 0, 0
Scheduled moves: 0, completed moves: 0
Bed compensation in use: none
Bed probe heights: 0.000 0.000 0.000 0.000 0.000
=== Heat ===
Bed heaters = 0 -1 -1 -1, chamberHWiFi: proterr: 0
eaters = -1 -1
Heater 1 is on, I-accum = 0.0
=== GCodes ===
WiFi: opterr: 0
Segments left: 0
Stack records: 1 allocated, 0 in use
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
serial is ready with "M122" in state(s) 0
aux is idle in state(WiFi: err: 0
s) 0
daemon is idle in state(WiFi: cachehit: 0
s) 0
queue is idle in state(s) 0
autopause is idle in state(s) 0
Code queue is empty.
=== Network ===
Network state is running
WiFi module is idle
Failed messages: pending 0, notready 0, noresp 0
WiFi firmware version 1.21RC1
WiFi MAC address 5c:cf:7f:2c:2WiFi:
4:9e
WiFi Vcc 3.42, reset reason Turned on by main processor
WiFi flash size 4194304, free heap 20240
HTTP sessions: 0 of 8
Socket states: 0 0 0 0 0 0 0 0
Responder states: HTTP(0) HTTP(0) HTTP(0) HTTP(0) FTP(0) Telnet(0)
WiFi: xmit: 0
WiFi: recv: 9
WiFi: fw: 0
WiFi: drop: 112
WiFi: chkerr: 0
WiFi: lenerr: 0
WiFi: memerr: 0
WiFi: rterr: 0
WiFi: proterr: 112
WiFi: opterr: 0
WiFi: err: 0
WiFi: cachehit: 0
WiFi:
WiFi: xmit: 21
WiFi: recv: 114
WiFi: fw: 0
WiFi: drop: 38
WiFi: chkerr: 0
WiFi: lenerr: 0
WiFi: memerr: 0
WiFi: rterr: 0
WiFi: proterr: 0
WiFi: opterr: 0
WiFi: err: 0
WiFi: cachehit: 0
WiFi:
WiFi: xmit: 4
WiFi: recv: 0
WiFi: drop: 0
WiFi: chkerr: 0
WiFi: lenerr: 0
WiFi: memerr: 0
WiFi: proterr: 0
WiFi: rx_v1: 0
WiFi: rx_group: 0
WiFi: rx_general: 0
WiFi: rx_report: 0
WiFi: tx_join: 2
WiFi: tx_leave: 0
WiFi: tx_report: 2
WiFi:
WiFi: xmit: 0
WiFi: recv: 0
WiFi: fw: 0

M552 s1
SENDING:M552 s1
serial: M552 s1
WiFi: mode : sta(5c:cf:7f:2c:24:9e)
WiFi: add if0
WiFi: scandone
WiFi: ../src/SocketServer.cpp(353): found network SUPERNET
WiFi: sleep enable,type: 2
serial: M105
WiFi: scandone
WiFi: state: 0 -> 2 (b0)
WiFi: state: 2 -> 3 (0)
WiFi: state: 3 -> 0 (4)
WiFi: reconnect
serial: M105
WiFi: scandone
WiFi: state: 0 -> 2 (b0)
WiFi: state: 2 -> 3 (0)
WiFi: state: 3 -> 0 (4)
WiFi: reconnect
serial: M105
WiFi: scandone
WiFi: state: 0 -> 2 (b0)
WiFi: state: 2 -> 0 (2)
WiFi: reconnect
serial: M105
WiFi: scandone
WiFi: state: 0 -> 2 (b0)
WiFi: state: 2 -> 0 (2)
WiFi: reconnect
serial: M105
WiFi: scandone
WiFi: state: 0 -> 2 (b0)
WiFi: state: 2 -> 3 (0)
WiFi: state: 3 -> 5 (10)
WiFi: add 0
WiFi: aid 8
WiFi: cnt
WiFi:
WiFi: connected with SUPERNET, channel 6
WiFi: dhcp client start…
serial: M105
serial: M105
serial: M105
WiFi: pm open,type:2 0
serial: M105
serial: M105
serial: M105
WiFi: ip:10.25.29.45,mask:255.255.255.0,gw:10.25.29.125
Wifi module is connected to access point SUPERNET, IP address 10.25.29.45
[/c]

thanks Ed