I have KDE 4.14.21 on Slackware 64 14.2. I have the KDE screen locker set to start in 15 minutes and require login after 1 minute [more]. I have the power management settings set to turn off screen after 30 minutes and suspend session after 20 minutes and lock screen. This all works fine except when I'm running a VirtualBox Windows 7 VM guest. Then the screen saver NEVER kicks in as long as the VM is running in the foreground. If another window is on top of the VM (e.g. Thunderbird, Firefox), or if the VM window is minimized, the screen saver does engage. This is a bit of a security risk as the desktop is left accessible if unattended. A shot-in-the-dark suggestion from the VirtualBox forum suggested there might be some setting in KDE defining how the screen saver works in regards to programs, e.g. playing a video.
The KDE 4 workspace is no longer maintained by KDE developers. If the issue is visible with Plasma 5.8, please add a comment or reopen the bug.
Christoph - sorry to hear that. KDE ships with a number of the major Linux distros and is the default desktop for Slackware. Does that mean it's no longer maintained by anyone? Is this bugs.kde.org site obsolete or now used for some other purpose? If not maintained here, is it maintained anywhere that you know of? I'm posting this question here because I don't know where would be more appropriate.
You could report it to the bug tracker of your distribution to find out if they still maintain the old version 4 of the KDE workspace. All major Linux distributions have updated to version 5. It is a bit unfortunate that you are using a distribution that has not yet updated, despite our maintaince announcements over three years ago. The bugs.kde.org bugtracker still accepts reports for recent versions.