The real culprit!
Terry Stough, October 13, 2017 - 10:47 am UTC
I can actually give the solution and I think this is a bug that needs to be addressed. I had set SEC_CASE_SENSITIVE_LOGON to false in a script that ran at the very end of my installation protocol.
I checked connectivity up until I ran that script and it worked fine. After running it, I could never again connect as SYSTEM, for example, in Enterprise Manager Database Express, SQL*Plus, SQLcl standalone 17.3 or SQL Developer 17.3.
Apparently in 12cR2, if this deprecated parameter is set, no passwords are accepted.
The problem script
Terry Stough, October 13, 2017 - 12:21 pm UTC
Sorry, I had not had my first coffee when I responded. The problem script was one written at 11g and that line did not get removed from the script. I probably did not read the Upgrade Guide because it was a fresh install.
I changed the response rating to Very Useful.
Thanks for your help.