Hello everyone !

I have a little issue with my plasma5 on Kubuntu 15.04. Namely transparency (for instance in kmenu, during apps movieng, etc) dispirited with no reason. Probably only what I have to do is edit some config files, because on another system profile, it works fine, and this issue is completely independent from desktop theme.

Can anyone met the same problem?

Can you check in System Settings whether the Desktop Effects are active?

Check your compositor settings, and see if it hasn't changed to xrender (after a plasma crash, perhaps?) Also, if you haven't done so, upgrading plasma via the kubuntu-backports ppa might help here.

I'm pretty sure it's this.
Apparently the transparency we know was a bug and never intended. The fact that the background contrast was there to, well, improve background contrast, but now actually worsens it because it introduces transparency makes no sense whatsoever, but what do I know.