Hey @FordGuy61 , thanks for sticking with this.
I agree, the obvious thought would be a permissions issue, but it has to be something that has changed in plex relating to those permissions, not to do with my own user/Windows credentials access to those drives.
The LAN folders in question are where I store all data and media, I have nothing on my local drive. I continue to access the files and folders directly within the same user account as my Plex server, and I can still access those folders and files through other software such as VLC, and on other devices such as my Android phone and Firestick TV.
My Plex install has used those LAN folders from the first day of setup, and though Plex hasn’t been without technical glitches, this is different in that it is the first time it cannot see the drives at all.
Whenever I have set up a new library, or made edits to an existing one, the folder drive letters are there at the first stage of selecting a folder. Now they are not, only C: and a usb data stick on J: show as available. I would expect that if it were permissions, the missing LAN drives should still show as existing and my I: M: and N: drives as they always have, but that if I tried to add them permissions problems would result in being denied access.
I followed your steps, the only exception being that I have already commenced debugging. I started your process, dechecking the auto clean and rebooting, just after midnight CET on 15th Feb if you want a timestamp to help you.
Thanks again
Nick
Ha! well I have no clue what this reboot did that several previous restarts didn’t, but we are back in business. I tested a film before starting this reply, still not working, but now, success. And all the drives are showing again.
Thanks again very much for your help and interest, very much appreciated. Our fridge is on the blink too, if you could just pop round and take a look that would be great. 
