T21P E2 firmware upgrade

[janp]

Hello,

I have a T21P E2 VoIP phone, which I would like to upgrade to a more recent firmware.
Current firmware is:
Firmware Version 52.80.0.130
Hardware Version 52.0.0.0.0.0.0

I downloaded firmware version T21P_E2-52.84.0.160.rom from Yealink support website.
I tried to flash the phone twice, but it didn't upgrade to a more recent firmware.
It stops at the message:
New firmware is installing... (on the phone LCD screen).

I left it idle for 1 hour, to not break the flashing process, but successfull flashing didn't work.

Do I need another firmware? How can I upgrade the firmware properly?