I have worked with a lot of software and have never seen such fragile software as AEM.
Installed AEM 6.1 JBoss Turnkey and everything was working except logic rules in the adaptive forms was missing. So began updating packages up to 6.1 SP2 CFP8 and the system slowly became a mess. First we couldn't log in with the administrator and we could only log in with the admin. Now we can not log in with the administrator nor the admin. We cannot access the CRX however we can login with Workbench as the administrator as well as the localhost:8080/adminui/secured/admin.faces.
JBWEB000071: root cause
Judging from your error - com/adobe/livecycle/usermanage/oak/loginmodule/LcCredentials - looks like some sort of upgrade error. The issues i see the most is when upgrades occur as oppose to fresh installs of a specific version.
I recommend opening a support ticket so Support can pass this issue to the AEM Form team.