Is the ram removable or soldered on? It could have worked its way loose and just need reseated?

If you're reseating, try cleaning the contacts with some isopropyl alcohol and a lint-free cloth. WD-40 CONTACT CLEANER *emphasises the contact cleaner part of the statement* is also good as it's basically spray IPA but it'll also take off any ink so be careful if there's something written on the board.

compaq ram test is also running fine (diagnostic tests)
now passing loops..

2Mb ram is soldered.
2 modules 2Mb and 4Mb are installed (removable)

Hm.. FFFF:0000 is shortly before the 1MB border, I'd day.
According to the web and other sources (PC Mag), this location is described as the BIOS reboot routine.
Or simply the location where a jump to the beginning of the BIOS happens.

so that space is occupied by bios ? (Shadow) maybe ?
maybe this is why ramtest is not finding errors ?

Did you try reseating it? The lack of errors combined with the no-show of the memory to the system says to me Hardware error rather than software. I remember I thought my last PC was dead and I was gonna cannibalise it for parts until and old house mate decided she wanted to play on it and when it didn't post, she reseated it all and it booted fine. "I would have played it but I didn't know the password" "password? It's dead!" "no, just dirty"