Hi Kamkar1, liamstears also has his K2S from Combi and he has successfully performed the update.
Also, Combi sent me K2S 27-03-20-FW-COMBI-20200306.zip and I still cannot get it to update with this :-(
I have emailed Rayman.Chan but no reply :-(
Do you have the pro cameras by any chance and need the pro firmware?
Or have you tried installing with nothing but power cable plugged in? I have my power plugged into EXT port