@phaedrux
Sorry for the delay, i was busy working the last days.

So my config looks like:
Pi 3 B+
external power supply (MeanWell)

It looked like everything is fine but on my last print i got the same error again.

here is the log:

Apr 29 18:03:49 duet3 DuetControlServer[374]: [warn] Bad header CRC32 (expected 0x2303ffca, got 0xecbd5173)
Apr 29 18:03:49 duet3 DuetControlServer[374]: [warn] Note: RepRapFirmware didn't receive valid data either (code 0x00000005)

Yes it lost connection, but luckily the print went on.
Does it buffer some of the gcode?

No, each Pi used another SD card.

I printed for 4h so far only one problem.
See attached picture

Sure, went back to the Pi 3.
I´m going to print the last failure part.

@phaedrux
I’ve measured 5,16V directly at the Mean Well.

-> Todays bad news: (Pi4 & external USB Power source)
the 2nd print was canceled, see attached pic.

Sorry to confuse you.

My starting configuration was:
Pi 4 & Mean Well RS-25-5
there were Problems with the SPI connection _ "Lost connection to DUET board, board is not available (no header)"

so yesterday i tried:
old Pi 3 B+ & USB Power Source (smartphone charger)
-> no problems so far

yesterdays 2nd try:
Pi 4 & USB Power Source (smartphone charger)
->although no problems

so my conclusion is:
the Mean Well causes Problems.

@phaedrux
I‘m using a Mean Well RS-25-5.
it only powers the Pi.

Sorry for double posting.

Result of 3 prints:
i´m sure the power source is the problem.
is there a way to smoothen the current?

@dc42
Pi and duet aren´t on the same DIN rail, so there can´t be any connection.
nevertheless: looked for overlapping screws -> nothing

next test:
Pi 4 with external USB Power source

Update:
used my old Raspberry PI 3 B+ and it worked fine for both testprints.

difference:
I used an external USB-Power source and the Pi isn´t mounted on the same place.

can anyone make sense of this?