What video card are you using ?
What driver for that card if any ?
What does your xorg.conf or 20-nvidia.conf look like ?
Identifier "Device 0"
Option "Composite" "on"
I deleted it but the problem remain
Under -> Desktop Effects -> Advanced
What does Compositing Type say ?
It should be OpenGL, you can try the slower XRender, then try the option "enable effects".
Xrander. When I try to switch it to openGL does not accept the change (I can't change the composing.)
Does it allow you to enable effects ?
Try ticking or unticking the following, play with them, see if any makes a difference
Enable direct rendering
I will try. [Edit. It doesn't work]
When was the last time "enable effects" worked on this machine ?
What changes have you made since then ?
Some weeks ago. I don't really remember
PS. Please note that an other account in the same laptop works correctly.
EDIT 2: The All Effects tab in Desktop Effects is empty.
Finally I solved the issue.
1st I installed libegl1-mesa-drivers
apt-get install libegl1-mesa-drivers
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
libegl1-mesa libgbm1 libopenvg1-mesa libxcb-dri2-0 libxcb-xfixes0
The following NEW packages will be installed:
libegl1-mesa libegl1-mesa-drivers libgbm1 libopenvg1-mesa libxcb-dri2-0 libxcb-xfixes0
0 upgraded, 6 newly installed, 0 to remove and 12 not upgraded.
Need to get 14.8 MB of archives.
After this operation, 38.1 MB of additional disk space will be used.
Do you want to continue [Y/n]?
2nd. I edited manually the .kde/share/config/kwinrc and I changed it to OpenGL. Now everything works as usual.