Newer Bseed Smart Wifi Socket not convertible anymore ?!? (sslpsk error)

Hi,

some time ago I bought some Bseed Smart Wifi Sockets ... and was happy that I could convert them with tuya-convert.
Because I was the first one, I also created a template for tasmota.

https://templates.blakadder.com/bseed_smart_socket.html

Now I bought some more of this devices, but sadly flasing is not possible anymore.

Saw the hint from the the page too late:

"Tuya-Convert might not be possible for this device if the manufacturer started using newest Tuya firmware since this template was published (2020-01-05)."

Someone edited the page and pointing me to #483

I read this thread and also found the Wiki:

https://github.com/ct-Open-Source/tuya-convert/wiki/Collaboration-document-for-PSK-Identity-02

But I dont understand, how I can help now.

Surprisingly I found an old device at home, that was never powered up before.
I tried to flash it with tuya-convert and got success.
During this try I got a Backup of the old Firmware.

Now I have 2 devices, one successfully flased, one that wont flash.
I have the Backup of the old Firmware ...
And now ... ?

Should I try to flash the old firmware to the old device and do an online update while I am sniffing the traffic?
Or is there any possibility to flash the old firmware (backup) to the newer devices?
Perhaps when I disassemble one and try with serial link?

I would be grateful for any advice.