Cause has been found (at least the main cause*) and it seems to be a combination of a change MS made when they implemented EFS (encrypted file system) in W10 and the way Sound Devices products write files that interferes with that change MS made. Latest word from the MS engineer posting in both the gearslutz and MS threads is that a patch is being worked on and will be introduced "as quickly as possible".
* so far the only reports of file inaccessability and/or corruption are from Sound Devices products so if you have encountered the problem on a file written by a different device please let the MS engineer know - his contact info is in both threads
note the MS thread linked in the OP seems to have more, and more current, info on the issue
Yes, it seems that Sound Devices was writing a Microsoft exclusive bit as "1", when it should have been "0".
Microsoft never used this bit in the past, so there was no problem - in W10, they use the bit as "1" to denote an encrypted file - which is where all the problems started.
Sound devices now have all the info, so they can correct the problem at their end.
Microsoft is working on a fix at their end and there should be a W10 update shortly.
They are also working on a fix that - after the W10 update comes out - will enable people with corrupted files to correct them.
Hats off to both SD and MS for getting to the bottom of this so quickly.