bootloader - Troubleshoot fastboot error 'remote: ' command not defined

Device: iMX 7ULP evaluation kit

Platform-tools: r28.0.3

Image: O8.1.0_1.4.0_ANDROID_TOOL_7ULP_GA (ARM 32 Android 8.1)

I used the image provided by nxp website on the device. Then I tried unlocking it.

According to a tutorial, I used adb to bring the system in bootloader via

> ./adb reboot bootloader

The device goes to the splash screen and stop there (Not sure if that's how it should be). adb does not recognize the device anymore and fastboot fails on any command as fowllows

> ./fastboot.exe oem unlock
                                FAILED (remote: ' unlock device failed.')
fastboot: error: Command failed

> ./fastboot oem device-info
                                FAILED (remote: ' command not defined')
fastboot: error: Command failed

> ./fastboot flashing unlock
                                FAILED (remote: ' unlock device failed.')
fastboot: error: Command failed

How can I troubleshoot that and unlock the device?