Same issue here. I tried to install iodéOS and /e/OS, but after relocking the bootloader, it would show that the phone is corrupt. I could still unlock and try again to no avail.
I figured I’d reinstall the stock OS to try relocking that way, but it still showed that it’s corrupt and now I can’t even unlock the bootloader. Guess I’ll contact support as well.
what exactly?
What have you done in which order?
First iodé?
Later /e/OS
What worked?
And when have you re-locked your bootloader? after whih OS installation?
Please a little mor precise.
I installed iodéOS and it worked fine, but features on some apps were disabled because my bootloader was unlocked, so I decided to lock it. Then it showed me this message: Your device is corrupt. It can't be trusted and will not boot.
I was able to unlock the bootloader again in fastboot and reinstall iodéOS, but locking the bootloader again showed the corrupt message.
I figured I’d try to install /e/OS to see if that worked, but the same thing happened. Worked before locking, corrupt after locking. This happens to many users according to the /e/OS forum: FP4 Device corrupted and won't boot in locked mode after fresh /e/OS install - Fairphone - /e/ community
Someone suggested that you need to have the stock OS installed before installing a custom OS for relocking to work, so I installed the stock ROM and tried to relock it. The result was the same: my device is corrupt and will not boot. This time, however, unlocking the bootloader doesn’t work anymore: FAILED (remote: 'Flashing Unlock is not allowed').
I managed to enter EDL/QDL mode, but looks like nobody has figured out a way to unbrick it through that yet.
So you did not locked it directly after you have flashed the OS and did a last wipe?
you have booted in the system without re-locking it first?
I think that’s your problem. iodeOS is signed for verified secure boot…
I thought that might’ve been the problem, so I tried to lock before the first boot. Still corrupted.
Have you seen this:
Would have been a good idea to read that first. 
It seems that thread links back here and the OP there luckily hasn’t bricked their device yet.
I am in the same situation after re-locking the bootloader : FP4 Root Access is possible, maybe a bit risky - #185 by cortomalese.
Look this post too Device not booting after flashing factory image and locking bootloader. I’m waiting for Fairphone support answer.
Yes, I did. The whole history is: to root my new FP4 I followed this :
So I start to unlock bootloader following this:
https://support.fairphone.com/hc/en-us/articles/360048646311-FP3-Manage-the-bootloader
The installation of Magisk went well and almost everything was fine for two months. I says “almost” because a bank app refuses to work, even with Magisk hide fonction, because of the unlocked bootloader.
To avoid reformatting my phone I disabled the authorization to unlock the bootloader in developpers settings. The apps bank don’t works anymore, but the phone is ok, until the last Magik’s update.
Other thing. When I tried to boot on TWRP image, I can’t wipe cache, because critical partitions was locked.
That’s when I saw that I couldn’t flash the original FP4 rom, I wanted to go back to the original state and re-lock the bootloader via fastboot flashing lock. And FP4 was blocked.
If I look at the /e/OS installation tutorial,
https://doc.e.foundation/devices/FP4/install
I see that not only the bootloader had to be unlocked fastboot flashing unlock, but also the critical partitions fastboot flashing unlock_critical, which the Fairphone manual does not say. I think that’s where my problems come from.
I think I have a point to make with Fairphone’s support: their tutorial don’t mention the need to flash critical partitions too. If “the Fairphone 4 (FP4) gives you the freedom to unlock your bootloader” it must also give us the resources to do so.
You linked to the wrong tutorial there, this is the right one:
https://support.fairphone.com/hc/en-us/articles/4405858258961
The instructions to unlock the critical partitions are in Part 2: Unlock your bootloader under step 7:
In a Command Prompt or a terminal, type: fastboot flashing unlock_critical and press the “Enter
” key
To be fair, those instructions haven’t been there from the beginning and they also only added…
If after returning to Fairphone OS, your phone can not boot to it, don’t lock the bootloader. If you have any issues with booting normally please contact customer support.
…after people kept bricking their phones.
So I agree, the documentation could (and should) be improved in many ways. But at the same time this is also the prize you sometimes pay as an early adopter, “hic sunt dragones”.
Doesn’t mean Fairphone shouldn’t do everything in their power to help the community fix it (especially if it’s their documentation that’s lacking) and I don’t mean “fixing” it with a replacement device offered at full cost… 
I think that’s the key point.
I assume that if you keep the authorization to unlock enabled (and you check at least once that your device can still boot after flashing the new OS) you should be safe locking it.
(and please don’t get me wrong - I don’t want to say you did something wrong and I’m really sorry for the state off your device! it’s meant for others planning to relock their bootloader…!)
I have more or less the same problem.
I wanted to try eOS, installed it according to their instructions and found it too much IPhone-like, so I wanted to go back to FairPhone OS and ran everything according to their instructions, then ended up in the Android setup process (so where you set up wifi, time, etc.) and turned off the phone and wanted to close the bootloader again to have everything as before … but then I also got this “Your device is corrupt. It can’t be trusted and will not boot.” and since then I’m stuck in the bootloader 
So would I have had to go through the setup process first or do something else? But there was nothing about that in the Instruction?
I have followed the instructions for eOS and Fairphone OS, that my phone now no longer works is a bit stupid…
will write to the support also… not even 1 week after my FP4 arrived
Just for the record: I have also exactly the same problem with my few days old FP4. So it affects a pretty high number of people. The device is stuck after flashing back to stock OS and relocking the bootloader. All attempts to reopen the bootloader via fastboot commands fail. I read a bunch of support threads and I think the device is simply bricked. It should be covered by the warranty since every step followed the official guides.
I opened a ticket at the customer support on 2022-03-17 and will report when there is an answer from their end.
iodé changed their instruction to flash their OS regarding this issue
They changed, and hopefully fixed it, by copy the iodéOS custom key as a root of trust to used by the bootloader to verify rom authenticity, and permits to boot when the bootloader is relocked.
So far, nothing negative has been heard about it in the beta TG group.
Instruction:
Just an idea: Have you tried making booting of slot a fail by removing the battery or a hardware during boot? Maybe you can get the slot-retry-count down by that to force using slot b. I know that this requires some precise timing and trial+error.
I had the same problem it is now solved
the costs are €29.90
From the first request to support (28.02.22) until I had the Fairphone back with new software (31.03.22) only 4 weeks passed, it takes time and can be solved, unfortunately only by the support at the moment.
Just for info
Did you get your phone back fixed or did they send a new one?
the fairphone was repaired
Have the same problem. Flashed e/os and locked the bootloader. No error appears. I used it one day but some bugs are killing the experience. The fingerprint sensor are buggy, the camera have only 12mp and the usb preferences don’t work and the usb c to 3,5 audio jack. So yesterday I decided to flash the stock ROM. Downloaded the lasted from the manual and flashed it. After booting the os run without issues. The last step is lock the bootloader. So I looked it in the fastboot. Reboot and run into the same situation. Thank for all the information guys. At least i wrote the support. The is nothing more to do…
I don’t know why something like that happen by using the official files? Maybe a wrong date in the phone? I only an android app developer and use adb the most time. But the hand full of people have the same bug. It seems to be a bigger problem that I think. I hope the Fairphone Devs find the issue.