FINALLY I may have found it:
Seems computer NAMES are half as powerful as IP addresses.
This can help quite some Win10 victims who are clueless after the changes in groups for computers.
So, type in the IP address to make a connection if the name in text gives no full permission.
With the cmd window and the command ipconfig you can find out the addresses of both computers.
You'll find a number like this: 192.168.70.5
Try opening one pc on the other computer by typing its ip address plus two backslashes proceeding the numbers, in a windows explorer window. In the example it would be \192.168.70.5
But every pc has different ip numbers of course.
Also don't use an internet browser: it must be windows explorer, you use to find C: etc.etc. And beware: \ and not //
If that doesn't work, you MUST add the same user with the same password on both PC's.
You gotta google that yourself. This means you must log in, on each PC with the same user and same password.
This greatly improves the chance they can see each other's shared drives with full access!
I sincerely hope someone will benefit from this info and solve the obnoxious Microsoft sharing problem.