ONE (of three) shares became inaccessible

Four in LAN, all are (and have been) at ver 2004.
Three are shared, and have been accessible immediately upon demand (no prompts).
They still are listed in Explorer. All firewalls still OFF for local.
Windows now fails access to ONE shared computer, giving the standard can't-do-it.
I verified that sharing permissions are unchanged.
It happens whether password protected sharing is off or on.

All three have the same three shares: C: and two first-level directories. Note that the failing computer gives access to one of those directories.
No amount of permission and ownership manipulation is of any help.

Today I fixed the other directory by recreating it (rename, create one of original name, move contents). Can't do same with C: of course.