There are unconfirmed reports that the patch breaks certain configurations of SMB file sharing:

https://arstechnica.com/civis/viewtopic.php?p=34406677#p34406677

This security update breaks SMB file sharing if you don't have the "Less secure" password setting turned on. If you don't have that setting turned on and try to connect to a patched Mac, your password will not be accepted. Quality work, Apple.