Difficulty in installing /e/ custom ROM after unlocking bootloader?

[AnotherElk]

@phonefrog

Could it help to go back to stock ROM due to the locked bootloader and then if stock is installed unlock bootloader again and install any custom ROM?

Installing the stock Fairphone OS requires unlocking the bootloader, too.
Can you unlock it in the current state?

For a while fastboot oem 8901_unlock could do it without the code procedure, but it seems this was removed …

/e/ OS Q (Android 10) for FP3/FP3+

The string 8901_unlock disappeared from aboot between Android 9 2.A.0120 and 2.A.0134, it is also not in aboot from Q-3.A.0054 so this wont work for Android 10. It would probably make sense to report the problem with the unlock website to the fairphone support. I only tried the webpage with the imei+serial from my FP3 and it also works fine (but gives different values for each request, even with the same input). If you are open for adventures, you could try to follow either of the two methods…