Not a good sign. This should work in almost any situation.
You didn’t leave the bootloader unlocked, by any chance?
In which case a rollback from 1.8.1 to 1.8 would probably be a matter of just switching the active OS slot back to the previous 1.8 one (Android rollback protection shouldn’t kick in if 1.8 and 1.8.1 are on the same security patch level).
In case the bootloader is locked (which is the default state of things on the Fairphone 3/3+), don’t unlock it now, because locking as well as unlocking it will force a factory reset for security reasons.
I think you could try safe mode to determine whether it’s an App at fault or the OS … [HOWTO] Reboot into Safe Mode.