Well, the Particle Update from the CLI works just fine. Something is going wrong with Particle Doctor, i tried it and got the same error described above. The P1 after this, is turned to Breathing White which means that the WiFi chip is turned off. After that, neither Particle Update works.
This time, instead using buttons to turn P1 in Safe Mode (breathing magenta) and after that a flash of Tinker using the WEB IDE, i tried to flash it using the CLI--> "particle flash --usb tinker". Success. P1 turned again in healthy state. Here is the message of CLI and i hope that will be helpful as in the two last lines something about invalid and valid is refered:
C:\Users\FOTIOS>particle flash --usb tinker
Found DFU device 2b04:d008
spawning dfu-util -d 2b04:d008 -a 0 -i 0 -s 0x080A0000:leave -D C:\Users\FOTIOS\AppData\Local\particle\node_modules\particle-cli\binaries\p1_tinker.bin
Copyright 2005-2009 Weston Schmidt, Harald Welte and OpenMoko Inc.
Copyright 2010-2014 Tormod Volden and Stefan Schmidt
This program is Free Software and has ABSOLUTELY NO WARRANTY
Please report bugs to firstname.lastname@example.org
Opening DFU capable USB device...
Run-time device DFU version 011a
Claiming USB DFU Interface...
Setting Alternate Setting #0 ...
Determining device status: state = dfuIDLE, status = 0
DFU mode device DFU version 011a
Device returned transfer size 4096
DfuSe interface name: "Internal Flash "
Downloading to address = 0x080a0000, size = 4740
Download [=========================] 100% 4740 bytes
File downloaded successfully
Invalid DFU suffix signature
A valid DFU suffix will be required in a future dfu-util release!!!