@kugiigi Anyway, I'm back in, thanks a lot for your tip :beaming_face_with_smiling_eyes:

@rapidrotator
This worked for me! I only had to alter the commands, as 'mount' would not show /system and the adb shell did not recognize the mkfs.extX -invocation of mke2fs :

OnePlus3:/ # cat /etc/fstab

Get the block device...

OnePlus3:/ # mke2fs -t ext4 /dev/block/<device>

As the /system-partition was already ext4, I wonder whether wiping the partitions will be enough and the trick is not ticking the 'first install' and 'wipe device' boxes....

Thanks a lot, anyways!