Another update:
I had a look into my Field Engineer Handbook and there the key combination to
abort the POST is Stop + A, which makes more sense than what is written in [1] on page 48.
[1]:
https://docs.oracle.com/cd/E19127-01/ult...561-11.pdf
So Stop + A to abort POST and Stop + D to force "a diagnostic power-on" (from Sun Field Engineer Handbook, Volume 1, dated 6/22/01, page "INTRODUCTION-8").
P.S.
Got back control over the E250. Unfortunately the spare NVRAM also seemd to have security mode enabled. So I went back to the original one and somehow I got it to forget that security mode was enabled. I still don't know if this was my doing (i.e. starting w/o NVRAM and inserting it during start-up multiple times) or if the NVRAM got its contents corrupted after a long enough time without the external battery.
P.P.S.
We should maybe ask the admins to move the Ultra2 debugging part into a separate thread.
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