Hi. I just wanted to post a resolutive update on the matter for future reference. As a matter of fact I discovered that my FAT PS2 had a modchip installed and that was the actual cause of conflict for freemcBoot. I just recently got this ps2 from ebay and the vendor didn't mentioned the fact that the console was chipped. why not? I would have paid even more for that. lucky me, I guess. too bad I had to got through all that 007 agent under fire nonsense...
The modchip installed is called "Titan SX". I believe it was quite popular here in italy back in the beginning of the modding scene, and it was later replaced by more advanced chips. There was no way to tell that the console had a modchip installed without disassembling it completely and I discovered it because after I heavily tweaked the CD diode in hope of getting the ps2 to play PSX originals, I was ready to replace the whole lens/drive block. Disassembling the console, I then noticed a wire going under the power board, then I remembered some of the remarks made in this thread about freemcboot not playing nice with modchips.
The strange thing is that, probably due to the relatively basic nature of this early chip, backups are booted only if the console finds the disc inside the tray during power up, the console being completely shut down (red light). If you get to the PS2 browser and then insert a backup or an original, you will get the RSOD.
Now it's clear that the modchip somehow messes with FreeMCBoot and prevents any form of media, being ps2 dvd's or psx cd's, to be played correctly from the browser. Now I can happily play psx backups booting without the freemcboot memorycard, or I can boot OPL from freemcboot and play games off the HDD.
thanks again for the support to everyone posted here