I can only guess be here goes:-
Somewhere the neutral was parted and was not a ring although likely the line was. When you did the work you also somewhere allowed the neutral to become disconnected so between the two points there is now no neutral.
When working on sockets testing is very important.
1) Test and note the loop impedance.
2) On removing socket test the ring is intact if not correct before going any further.
Note:- Clearly if not intact after completion you will expect lower loop impedance readings than at start but if it is intact the same readings will be measured.
3) Do the work.
4) Retest see note so even if you were to get a wire come off as last socket was replaced the loop impedance figures would alert one of the errors.
OK now to the DIY world where you don't have a loop tester. Again with power off main isolator not a MCB as the MCB does not disconnect the neutral so you can get wrong readings.
With a multimeter test both there is continuity between earth pair of wires i.e. earth, neutral and line and also there is no connection earth neutral and line. Until you get correct readings do not proceed with any alterations.
It is likely when you check the two neutral wires you will find no connection and you will need to remove sockets to find the fault. You can take a guess one fault where you have worked last socket in the run and other one last socket working in the other direction.
It will be a long job. In theroy without the special meters an electrician uses you should not do the job. In practice yes you will do it but it will take a lot longer.
There are things you will never find with a multimeter our insulation testers use 500 volt to test and the RCD tester will measure milliseconds it takes for a RCD to trip. A plug in tester like this
with loop test will show many more faults than you are likely to find with a multimeter but you will still need the multimeter to find the rest.