Faulty wire on ring main tripping fuse identified, but what could cause it? (UK)
You have taken the right steps to trace and identify the area of the problem, and the nature of the fault, but you still cannot know the source of the fault without further inspection.

Many things, including those you have identified, can cause faults like this. Yes, burying a junction box in the ceiling is inconvenient and inappropriate if it is not accessible for servicing, but it is still very common. If there is a junction box between the two sockets you have disconnected, the fault could be with or within it. If not, the fault is certainly in the cable.

Examples of what could cause continuity between live and earth wires in a multi-core cable are mechanical damage, insulation breakdown, insertion of a conductive item into the cable such as a nail or screw, loose, poor or incorrect connections at a junction or other accessory, presence of conductive liquid, etc.

The simplest off the cuff remark would be replace the run between the two sockets, but if this means removing tiles, chasing brick or plaster work, redecorating, lifting floorboards - it can become anything but simple.

If you can observe the cable within the ceiling, such as by removing a spotlight and looking in the hole (with a mobile phone camera, for example), or lifting floor boards above, you will be able to see whether there is any other damage or signs of damage (such as scorching, water marks) to the cable in question or the area nearby. If the fault is in that area, you have a chance of fixing it.

If however it's a nail through the cable on a wall, and it's installed in capping and clipped beneath the plaster, the only solution will be to remove it destructively, for which I recommend you contract a suitable electrician.