Ok, thanks to the inspiration of the two of you, I got it done. It was actually quite easy, after I found the well hidden encryption settings.
I have read many tutorials on the internet, some very complicated and many pages long, but basically it was a simple 3 step process, which I would like to summarize for people that come here for help/reference:
1) On Win 10 box install SMB 1.0 feature. Run Appwiz.cpl and click "Turn Windows Features On Or Off" and check everything that says "SMB 1.0/CIFS" (NOT "SMB Direct"). - thanks to x16wda
2) Assign fixed IP addresses to both computers in the same private network. Put any Gateway address from the same network, so that the Network will be recognized as private by the OS, and not public.
3) Open Control Panel > Network and Sharing Center > Change advanced sharing settings > click on "All Networks" and activate "Enable file sharing for device that use 40- or 56-bit encryption". (inspired by Shades comment)
Now one can share folders between the XP and WIn10 computers.
Example for static IP address setup (thanks to Shades):
192.168.1.10 192.168.1.11 (IP address)
255.255.255.0 255.255.255.0 (Netmask)
192.168.1.10 192.168.1.11 (Gateway)
192.168.1.10 192.168.1.11 (Primary DNS. Optional, because machines are not being used to access the internet)
192.168.1.1 192.168.1.1 (Secondary DNS. Optional, because machines are not being used to access the internet)
Hint: If you want to easily save and later revert to your initial TCP/IP options, the wonderful tool NetSetMan
is extremely helpful. Free for private use.
Thank you so much for your help!