应该去哪看 log 呢？
我看到 kde4 好像发生过类似的事情 permalink.gmane.org/gmane.linux.debian.user/501285 后来修复了……
记住时间，然后 journalctl 一下。su 以后 journalctl 可以有更多信息：-）一般都会有的，我使用 kde 钱包都有日志的。
怒用 google 搜到了……
bugs.kde.org/show_bug.cgi?id=337470
原因 ]
/usr/lib64/libexec/kcheckpass
Password:
Information: Permissions on the password database may be too restrictive.
Authentication failure
解决 ]
The problem seems to be with the permissions on /etc/shadow
chmod o+r /etc/shadow fixes the above issue, however this file should not be world readable
=========
Alternatively, chmod +s /usr/lib64/libexec/kcheckpass temporarily fixes it until plasma5-workspace is reinstalled.
最后我使用了 +s 的方法暂时性解决
chmod +s /usr/lib64/libexec/kcheckpass