Welcome to the club.
Being afraid by the security warning, I locked the bootloader after having had installed Ubuntu Touch.
Solution
Contact the Support via Chat and ask them to create a return request for you.
Unfortunately, it seems that installing a third party operating system (OS) has damaged critical system files on your Fairphone. This means that standard commands cannot be used to unlock it or install another OS.
The good news is that we can fix this in our repair center in France. We have access to special tools which allow us to install new software even on a locked device. Keep in mind that since the damage was caused by a third party OS, this repair will not be covered by your manufacturing warranty.
The costs will be between €25 - €35 for Assessment & handling costs. Shipping costs are separate and vary by country (Read more about shipping costs).
I paid €50 including shipping from Germany to France via UPS.