Recently I discovered that I could no longer access my Windows 7 computer from the other computers on my home network. At first I thought it was something to do with the username or password, but the behaviour when these credentials are entered wrongly is very different from the error I was getting, which basically said I didn’t have access and that there was an “Invalid Handle”.
I eventually tracked down this webpage https://borncity.com/win/2019/01/09/netzwerk-issues-with-updates-kb4480970-and-kb4480960/ (which references this article by Microsoft https://support.microsoft.com/en-us/help/4480970).
After adding the entry mentioned in this article into the registry, and rebooting, suddenly all my shares (folders, printers, etc) are accessible again! So it seems it was something to do with the January 2019 update.
I am adding this blog post, mainly as a reference for myself, in case it happens again and it took a while to find those web pages, after trying various searches.
In the registry, under the key HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\system add a DWORD value called LocalAccountTokenFilterPolicy with the value of 1.