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 described here (maybe fastboot first and then the hidden edl version):
And I have one more option: If you can get your wifi router (DNS server) to redirect factory.fairphone.com to fp3.lrnz.org (or 148.251.41.172) instead, try unlocking with an arbitrary (non-empty) unlock code while it is connected to that wifi. Opening https://factory.fairphone.com in the browser on the phone should then give a certificate error (e.g. ERR_CERT_COMMON_NAME_INVALID).
Disclaimer: This will probably send your IMEI + Serial to my personal webserver but I promise not to log those requests.