I'm sorry if I just answer now, but I've been busy with work and had to put this matter on hold. So I try to answer the questions in order:
1) I used the gerber present in the ryzee119's github, I think they are the most recent. Here is a photo of the LPC header, sorry for the low quality:
https://imgur.com/a/K6UtAuC
I have already tried to do a reflow, but nothing has changed. Also, I would like to point out once again that I have no problem with an Aladdin Xblast.
2) I used all the files I could find. I tried with the dumps present here on the furum, I tried with the official files present on Xbins. Everything behaves exactly the same way.
During my research, however, I found a very strange thing. After flashing the bios of a XeniumOS gold (but it is valid for any other version), I started the xbox and as soon as XeniumOS loaded, I turned it off. Subsequently, through xenium-tools I made a dump of the contents of the modchip and compared it with the file that I flashed. To my surprise, I found that the dump is completely empty. It is as if the modchip's flash is completely erased after the first boot. This would explain why on the second boot the console goes into FRAG. However, if I flash XeniumOS with Xenium-tools and then immediately dump the flash, they are identical. So something is happening at boot that erase the flash. I just can't explain it...
See image: https://imgur.com/a/NkxyIUO
Also, I don't know if it is a normal behaviour, but when I flash XeniumOS and reboot the xbox, before loading XeniumOS, it tooks about 5-8 seconds to boot during which the led in the modchip is first amber, then it turns blue. If I reboot it's red.
EDIT: I'm reading here and here that if XeniumOS detect that a modchip is a clone, it will lock up the chip...it's basically the exact same thing happening to me. It's very strange...have you some ideas @Ryzee119?