在我安装显示管理器之前，我的等离子桌面环境运行得很好。但是kdm在我安装kdm并重新启动我的笔记本电脑后，Gentoo 启动到黑屏，并带有一个白色复选框，这表示
Could not start kdeinit5, Check your installation.
然后我点击了okay按钮。它掉入 tty 并带有一些错误消息。
xauth: file /root/.serverauth.8002 does not exist
X.Org X Server 1.18.4
Release Date: 2016-7-19
X Protocol Version 11: Revision 0
Build Operatin System: Linux 4.4.39-gentoo #1 SMP Fri Jan 13 20:24:54 CST 2017 x86_64
Kernel command line: BOOT_IMAGE=/boot/kernel-genkernel-x86_64-4.4.39-gentoo root=/dev/sda2 rootfstype=ext4
Build Date: 13 January 2017 11:03:46PM
Current version of pixman: 0.34.0
Before reporting problems., check http://wiki.x.org
to make sure that you have the latest version.
(==) Log File: "/var/log/Xorg.0.log", Time: Sat Jan 20 21:12:01 2017
(==) Using config directory "/etc/X11/xorg.conf.d"
(==) Using system config directory "/usr/share/X11/xorg.conf.d"
The application failed to start because it could not find not find or load the Qt platform plugin "xcd" in ""
Available platform plugins are: wayland-org.kde.kwin.qpa, minimal, offscreen, wayland, xcb.
Reinstalling the application may finx this problem.
startkde: Starting up...
/usr/bin/kdeinit5: relocation error: /usr/lib64/libQt5Gui.so.5: symbol _Z10qAllocMoreii, version Qt_5 not defined in file libQt5Core.so.5 with link time reference.
startkde: Could not start kdeinit5. Check your installation.
xinit: connection to X server lost.
waiting for X server to shut down (II) Server terminated successfully (0). Closing log file.
然而，在我安装之前它运行得很好kdm。不知道为什么安装后就闪退了kdm。
我也从互联网上找到了一些建议并进行了尝试。但这些都不起作用。任何人都可以提供一些解决方案或一些线索吗？我的环境是：
等离子版本是：plasma-5.26.0
kdm 版本是：kde-plasma/kdm-4.11.22
内核版本是4.4.39
答案1
首先，感谢@StefanR 节省了我很多时间！
正如上面@StefanR所说，这个问题是由Qt版本不匹配引起的。在我删除了kdmbyemerge -C kdm并更新了一些qt由引用的内容后emerge -av @qt5-essentials @qt5-addons，我的 kde 桌面环境再次发挥了魅力。
必须提及
如果你的 portage 声称 There are no sets to satisfy 'qt5-essentials'或者There are no sets to satisfy '@qt5-addons'，你的系统中可能缺少某些层，可以使用以下命令解决：
emerge layman
layman -S
layman -a qt
layman -a gentoo
emerge --ask @qt5-essentials @qt5-addons