Hello
I have
Kdenlive 16.12.3
on
Linux homeserver 4.11.8-200.fc25.x86_64 #1 SMP Thu Jun 29 16:13:56 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
under XFCE

Open or save dialogs appers as transparent windows : https://ibb.co/mDLOpa
Other KDE applications works fine.
Please advice something?

In settings somewhere (I can't check, I'm not on Linux at the moment) - make sure you have Opengl selected. That's the only thing I know of that can cause that, but I use a different version of Linux (KXStudio) so it may not solve your problem.