It may be buried in here somewhere but thought I'd throw this out there just in case or to help folks that haven't found a solution here.
I was having trouble suddenly with several folders shared between my win 7 and 10 boxes. As far as I know they had worked before and suddenly I was getting you need permission errors from the 7 to 10 box, and at least one from the 10 to 7 box.
Poked around here, looked at my security settings, tried a few things but nothing worked. Then I remembered that some folders need a Everyone box with at least the basic 3 permissions set. I checked a working folder and it had Everyone with all boxes checked, checked a non working one and Everyone was not there. Pretty sure it HAD been there but it wasn't now, added it to all the bad folders and now both boxes are again happily sharing the drives and folders I picked again.
Like I said it's probably already in here somewhere but a good reminder for anyone having weird file sharing problems with only a few folders, yet all the others work.