When I suspend the desktop effects with Shift+Alt+F12 and then resume them the plasma panel and krunner are no longer transparent and blurred (I use the Air theme) - they look like as if compositing is not enabled, but it is. Is this a bug in Plasma/Kwin, or am I missing something? How can I work around this issue? Thanks in advance!

Edit: sorry I forgot: I use KDE 4.6.2 with the nouveau driver.

Ok, I found this topic: viewtopic.php?f=67&t=93226 which seems to be similar. However, in my case after I restart Plasma (i.e. logout login) the Air theme is ok - it's transparent and blurred. Could it be that "pixmaps" (I'm not sure if this is technically correct ) are cached and not updated after desktop effects are resumed?

Well, I found it! After digging around, I found that the following files are being changed on suspend/resume of compositing:

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/var/tmp/kdecache-$USER/plasma_theme_default.kcache
/var/tmp/kdecache-$USER/plasma_theme_internal-system-colors.kcache
/var/tmp/kdecache-$USER/plasma-svgelements-defaults

However, I noticed I have an old lock file:

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/var/tmp/kdecache-$USER/plasma-svgelements-default.lock.ct4176

I removed it and now the plasma panels are ok after suspend/resume of desktop effects. But krunner is not - it's still non-transparent on resuming the effects. Perhaps, it uses some sort of cache, too??

Yes, they both use the same caches. When this happens to me I usually log out, switch to a virtual terminal and use that to delete the entire /var/tmp/kdecache-$USER/ directory.

I removed the KDE cache folder:

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sudo rm -r /var/tmp/kdecache-*

Then I re-enable the blur plugin from Desktop Effects.

It magically worked!
Thanks folks, you saved me from a fresh installation!

I can regenerate the problem by removing ~/.kde and restarting Kubuntu.
Drivers: fglrx | ATI Radeon
KDE 4.8 on Kubuntu 12.04.3