Hi guys!
I recently aquired beautiful HP Kayak XA-s 6/350 MT (also known as XA-s 02xx). It originally came with single Pentium II 350MHz CPU, but as the name implies, it is (or at least should be) scalable. The BIOS was in German, so using an archived BIOS update, I was able to get it to the newest version (HK.11.11 - from 2000) in English.

Unfortunately my luck ends here. Upon installing Pentium III 500MHz (Katmai) CPU, the system does not boot. Front LCD says "Error! BIOS".
I've tried two different known-good Katmai CPUs, the installed VRM is in spec and capable of running Katmai, all the switches were set correctly, 2nd slot terminator is OK.
I did a Memtest86 pass on it, no errors. It also succesfully ran Windows 2000 for a while. I have tried Klamath 266MHz and it worked fine. The only issue seems to be Pentium 3 😒

What do You think, what should I do? I also found a littlebit older version of the BIOS - the HK.11.09 from 1999. This poses a second question: should I try and rollback?