PS2 Slim (V12) black screen on PS2 boot, PS1 games work

Recently I picked up a SCPH-70004 (V12) PS2 that's causing me some headache today.
It had a busted laser as they often do.

So I did my usual overhaul - replacing the laser, ribbon, summ0ne fix and Modbo 5.0D chip.
The PS2 starts like normal. If I insert a PS2 game (CD or DVD), the drive reads the disc fine, but instead of the Playstation 2 logo I just get a black screen.
PS1 games work fine.
However if put the modchip in fast boot mode, PS2 games start too!

I did some troubleshooting like checking all the connections and swapping the chip with a different one, then desoldered everything alltogether (including the summ0ne fix) but the problem didn't change.
I am very confident in my soldering work, so I'm 99% certain my install didn't cause the issue.

Anybody heard an issue like this before? What could cause a lock-up that can be avoided by using the fast boot mode?

 

PS2LOGO would read in the game's PS2 logo (stored within the first 12 sectors of the disc). Decryption is done by the hardware.
I guess "fastboot" skips the PS2LOGO program, hence this problem is evaded altogether.

 

That's interesting, so maybe those sectors aren't being read properly? Or the decryption fails for some reason (bad solder joint/connection, data corruption where the PS2LOGO program is stored)?
I've heard about the RS2004FS driver IC going bad. However that IC just does the mechanical control and the drive looks and sounds fine during gameplay.

 

If it's related to PS2LOGO itself, then maybe you want to check the region arond the address & data lines that you soldered the modchip to - in case something was accidentally bridged there.

I have this feeling that the DRM works fine because the browser didn't appear again. The browser obtains the boot filename from sceCdReadKey. It also reads SYSTEM.CNF for the BOOT2 parameter, but that is used to verify that the filename read with sceCdReadKey is correct. Failure to read that should result in the browser being booted again.