Try setting up another admin user account to see if the same problem continues. If Back-to-My Mac is selected in System Preferences, the Guest account will not work. The intent is to see if it is specific to one account or a system wide problem. This account can be deleted later.
If the problem is still there, try booting into the Safe Mode. Shut down the computer and then power it back up. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart. For each disable/delete, you will need to restart if you don't do them all at once.
This worked for me.
A caution for people like me who are unaccustomed to using Terminal: Linc is correct that when you type in your password, it won't show. In fact, nothing shows - I thought Terminal was just not responding. There was no acknowledgement that anything had even been typed. I had interpreted Linc's words as meaning that the password characters would not show - not that NOTHING would show. I'm not blaming Linc - he was literally correct, and it was my interpretation of his words that caused my confusion. But I suspect others will make the same wrong assumption I did, so pay attention.
There was also no sign that terminal was churning away on the command. It sat there unchanging and mute until it finally spat out the line with the "$" at the end. When I returned to the system the problem appeared to be fixed. Apps that displayed the first-use intro screen upon every start up now stopped doing so. I'm hoping this will eliminate several other problems related to password and permissions that DIDN'T show up when doing a check of permissions using the keychain app.