Can't erase cache using fastboot on OnePlus2

Question

My OnePlus2 (I think using OxygenOS) displays start icons (normal logo and fastboot, depending on how I boot it), however, afterwards the screen always remains black, while the phone responds to key presses.

Booting in recovery mode starts up the phone too, but never displays anything.

I tried to clear the cache using fastboot (following the fastboot-info on here), but get this response:

$ fastboot erase cache
******** Did you mean to fastboot format this ext4 partition?
erasing 'cache'...
FAILED (remote: device is locked. Cannot erase)
finished. total time: 0.020s

When I tried to unlock, following the -help suggestions, e.g.:

  flashing unlock                          Unlocks the device. Allows flashing
                                           any partition except
                                           bootloader-related partitions.

But it doesn't work:

$ fastboot flashing unlock
...
FAILED (remote: unknown command)
finished. total time: 0.006s

And another command I read about seems disabled:

$ fastboot oem unlock
...
FAILED (remote: oem unlock is disabled)
finished. total time: 0.009s

Is there another way I can erase the cache using fastboot or adb? I hope clearing the cache might resolve the black screen issue on my phone.