Passwords suddenly stopped working for several but not all members...

Over the past several weeks I have had at least 4 members not able to gain access to my members sections using a username and password that had been working. When they submit their username/password they now get an HTTP 500 error. Changing their password does not work either. I can create new credentials for them using a new username, password and email that does work.

I'm using:

Joomla 3.6.4
CB 2.0.15
CB Paid Sub 4.1.0
PHP Version: 5.4.45

Any thoughts?

500 error is a fatal server error. You either have some PHP or SQL that is failing. Enable debug mode and maximum error reporting in Joomla global configuration then retry to see what error is being output. If none check server error log.