I tried now to upgrade a second board and it is more or less the same behaviour.
First I failed on a RB450 now on a RB1000.
It was the same, I upgraded from 3.x -> 4.17 (ok) -> 5.26 (ok) -> 6.42.5 (fail)
On the 450 I uploaded the npk via the web interface now I tried to use winbox
but the symptom is the same after the 6.42.5 it will just go into a boot loop and continue beeping.
I could not find any update path info. What I did not do is any license update. After I had recovered
the system the first time I simply used winbox for that. What should be the right / fastest way to get
an updated router board without (I have physical access to the devices) much issues?