in reply to muesli

When the ethernet cabling in the second room has an issue (like broken or swapped cables) you might get something lime that.

But you seem to have tracked it down to the power supply, so everything seems OK now 😉

We once had machines boot without issues in one room but with problems in another room. turned out it was the room lighting in one room that was emitting IR frequencies that interferred with the machines... Took us a while...

in reply to muesli

Have you tried a shoot-out regarding connections?
I.e. powering it from the "working" place with long cables (including betwork, monitor and peripheries).
Does it boot completely standalone (only Pi+Monitor, the latter powered with a long mains cable from "working"), pi running from a USB power bank?
If so, power for the monitor from the flaky location mains. Then the pi. Add one connection for each consecutive boot to find the culprit. (My guess: network connection)