Hi, again i come with problem. I have Arch Linux x86_64 with KDE Plasma 5. I use Breeze theme. This is my problem:
When i turn on my PC KDE panel isn't transparent - i have enabled Compostitor, but my panel has solid background without transparency. Other desktop effects works without problems. I can fix this problem by turning off compositor, remove directory /var/tmp/kdecache-user_name(this directory contains only one file - icon-cache.kcache) and turn again compositor. After that panel has transparent background and works without problem. But when i turn off my PC and turn it on the issue returns and i can again fix it by the same way. I tried change Render from OpenGL2.0 to OpenGL3.1 and XRender(when i choose XRender my panel becomes solid without transparency) - nothing works. How can i fix this problem once and for all? I tried to purge my KDE configuration(i removed .config and .local directories) but this won't work. English isn't my first language, so please excuse any mistakes. Thanks you in advance for help.

have you checked https://wiki.archlinux.org/index.php/KDE
there's a few suggestions about transparency/compositing.

how did you install kde?

I installed KDE from repository by pacman. In Arch wiki i found nothing interesting. As i see last time - kwin restart(kwin_x11 --replace command) solve my problem and transparent back. I solved my problem by the stupid way - i added kwin restart command to autostart, but i want to know how i can solve this problem by normal way.

what else is installed? maybe something else uses compositing, somehow getting in the way?

how do you start kde?
what was installed before you installed kde? a fully functional command line system? or something graphical?

the problem with archlinux is, there's so many things to troubleshoot at this point, and only you can know how to do that, and what exactly to do, because it was you who set up your system in the first place.

No, i don't have apps, that uses compositor. Only docky but before i installed docky problem exist. Before i installed KDE i had only command line core system, after i installed Arch(from console, i didn't used graphics installers) i installed Xorg and KDE. If you need to know more, please ask.

Edit: Very interesting... Problem exist when i using linux-lts kernel(version 4.1). When i boot Arch with normal Linux kernel(actually 4.2.5-1) problem isn't exist, and panels are transparent. So, i think problem is solved now, thanks for replies.