Well, if the machine is equipped with Basic 4.0, the address $c5b0 should be the
entry point of a string processing routine.
The data you've provided can't be interpreted as a working ML routine, it contains
invalid instruction codes, and break instructions, preventing the machine from boot.
Even if I'm wrong in regard of the Basic version, this piece of code won't run.
This is how it looks when translating the numbers to Assembler:
- Untitled.jpg (24.18 KiB) Viewed 18121 times
I've marked the critical parts with arrows.
So the program execution is not possible from $c5b0, due to the errors.
My guess is that the machine crashes during displaying the power up message,
which needs this string processing subroutine.
I think your first hunch was right, the Basic ROM contents are seem to be corrupted.