Fairphone stuck in fastboot mode - bootloader unlocked

If you’re going to install FPOS as a clean install and you are running in the same problems then me, do two things:

before you begin with the FP script in bootloader
fastboot erase avb_custom_key
fastboot --disable-verity --disable-verification

Then modify the flash_fp4_factory.command script:

# Toggles
AUTO_REBOOT="true"                 # Control reboot behavior. Default is "true"
DATA_WIPE="true"                   # User data wipe. Default is "true"
DRY_RUN="false"                    # No-flash run. Default value is "false"
FRP_WIPE="true"                    # frp wipe. Default is "true"
INTEGRITY_CHECK="true"             # Checksum run. default value is "true"
REBOOT_TO_BOOTLOADER="false"       # default value is "false"

→ set the FRP_WIPE varaible to false

Then the script should run trough and FPOS should boot.

Else, your bootloader is damaged, as @hirnsushi already mentioned.