Hi Vikas, Jonas.
Thank you so much for your advise! I have reload the R18.4R3 and now everything back to normal.
Thanks again!!
Actually, I have no intention to upgrade the firmware to R19.4R1, however when I click the "Support -> SRX Series" and select SRX340, the Juniper portal page will display the "19.4R1 firmware" for download - and with the description box: "SRX300 & SRX500 - Series"
Anyway, your help and support is highly appreciated.
Best Regards
Matthew Ho