The only reason for that was, that I could not find the boot.img for A.170.
Now I downloaded the huge FP4-FP4G-factory.zip.
The unpatched boot.img is images/boot.img, right?
Can I repeat fastboot boot boot.img with that file or does that cause damages?
I just checked the boot.img I uploaded, and that’s working just fine. Any particular reason you are using Magisk 25.1 instead of .2? Maybe that’s causing issues, since it’s been patched with the newer version 
You can absolutely boot the stock boot.img, but if you don’t patch it beforehand, all you get is the same experience as if you would have booted into your normal system.
You wrote that you patched the A.170 you provided with Magisk 25.1. That’s the only reason.
25.2 makes no difference.
I did, where? Last 25.1 image was A.142.
Allright, sounds like a fastboot issue then, you almost certainly don’t end up in a rooted userland for some reason.
What output do you get when you fastboot boot any of those images? Have you tried different USB-cables / -ports? fastboot can be a bit particular when it comes to USB connections …
Meanwhile I successfully rooted the phone using the boot.img from the factory image patched myself.
Thank you for your assistance,
You did not, I have read wrong. Everything o.k.
Great you managed to make it work 
Out of curiosity, did you change anything else, apart from patching it yourself? I’m not sure what went wrong here
No. Mainly I processed along Fairphone 4 – Root -December 2021 – Stijn D'haese the 2nd time. Since it’s the first time I’m doing it with a new phone, it’s better to have it described in more detail.
I just installed the .175 update. After point 3. the phone seemed to be stuck in a boot loop (repeating display of the warning re. unlocked boot loader). After a few minutes the phone started to normal operation. But “About–>Build number” says: FP4.FP4G.A.170.20220920
Therefore I assume the update has not been applied. What shall I do?
Sounds like failed attempts to start the active slot and after defined number of failed attempts the system fell back to the backup (previous) slot.
So I think there’s a problem with the updated slot.
O.k., but what is the solution?
First of all, if you aren’t up-to-date on your backups, check those now.
Then I would try installing the OTA update again, pay attention to any error messages you might get during that process and while reinstalling Magisk as well.
If possible start a logcat before the update, adb logcat -c && adb logcat > logcat.txt.
Backup is o.k.
The failed update left me at .170 without root.
I downloaded an unpatched .170, patched it with magisk and transferred the patched file to the PC.
Then I continued with “adb reboot bootloader”. That leads to a reboot into the bootloader showing
START
Press volume key to select, and press power key to select.
Fastboot Mode
Your action list to root the phone has “temporarily boot from the patched boot.img fastboot boot /path/to/patched_boot.img” as next point.
If I issue “fastboot boot <magistk_patched-25200_…img” the only I see on the shell (PC) is
< waiting for device >
“fastboot devices” shows nothing.
What’s wrong? Did I miss something?
Usually a USB-cable / -port issue, fastboot can be picky. Try different combinations until the phone gets detected.
If you are on Windows, might be a missing driver as well, check device manager with the phone plugged in and in fastboot mode.
I used the same cable like at the first rooting. System is Linux, adb devices recognizes the Fairphone until “adb reboot bootloader”.
I pressed no key in the bootloader screen. I hope, that’s o.k.?
At the end of magisk_log_2023-01-05T18.56.06.log there is:
--------- beginning of system
01-05 18:55:36.163 18741 18741 V ActivityThread: handleTrimMemory:com.topjohnwu.magisk
01-05 18:55:39.582 18741 18741 I AppCompatDelegate: The Activity's LayoutInflater already has a Factory installed so we can not install AppCompat's
01-05 18:55:40.852 18741 18887 W CursorWindow: Window is full: requested allocation 30 bytes, free space 12 bytes, window size 2097152 bytes
01-05 18:55:50.684 18741 18741 I AppCompatDelegate: The Activity's LayoutInflater already has a Factory installed so we can not install AppCompat's
01-05 18:55:50.936 18741 18951 W CursorWindow: Window is full: requested allocation 30 bytes, free space 12 bytes, window size 2097152 bytes
01-05 18:55:51.206 18741 18755 W System : A resource failed to call release.
01-05 18:55:51.206 18741 18755 I chatty : uid=10217(com.topjohnwu.magisk) FinalizerDaemon identical 1 line
01-05 18:55:51.207 18741 18755 W System : A resource failed to call release.
“Window is full” – concerning?
Next try: It has been an issue with the USB port. Using an other USB port let fastboot recognize the phone. Re-rooting 170 worked. Performing the OTA update doesn’t.
After the process the phone is stuck in a boot loop until some “max try” counter booted the old 170 – again without root.
Now I’m out of ideas…
EDIT: Stand ist jetzt: 170 mit root.
Those are only classified as Warnings, so that’s probably not it.
Any other errors in the logs or in the information the Magisk install screen outputs?
The OTA installation completes successfully, right?! Have you tried rebooting into an unpatched 175 release?
We might have to consider one of the Magisk modules as a cause as well, which ones do you have installed at the moment?
Did not notice the letters (I)nformation, (W)arning… Thanks for the hint.
Yes, I think so. At least there were no error messages. “Optimizing system” took quite long.
That would require to reboot after the update. Your OTA process says Install OTA-update and don’t reboot.
Not many: Systemless Hosts, Zygisk - Clipboard Whitelist
What happens, if I disable these? Like “not installed” or just “out of order”?
You’ll end up with an unrooted system, but we’ll at least know if it’s the OTA update or Magisk breaking the update.
If it boots properly you can just use the 170 boot.img to re-root it, other than no connectivity and some warnings that usually works just fine.
Those probably shouldn’t cause issues, the Zygisk one might
No harm in disabling them, but I’d start with booting 175 first to narrow down the issue. If that succeeds and Magisk still fails, the modules would be the next step.
I shall
Is there a risk bricking the phone with this?