Ok, PITA this one. Open up any media player with an mp4/mp3 file and get an immediate hard freeze; I have to ctrl+alt+del to try to log off which triggers a Sorry Plasma error message
KDEInit could not launch '/usr/lib64/libexec/kf5/kioexec'
Which I close out and everything reverts back to normal.
I’ve had the message flash up before but it never really adversely affected anything.
Cannot reproduce issue with any media player and mp4 here on Leap 42.1 with Plasma 5.
Re PITA: pain killers recommended or seek medical advice asap. 
Found the workaround in the KDE bug report for it : 345381 – Folder widget - KDEInit could not launch '/usr/lib64/libexec/kf5/kioexec'
Found out this only happens if the desktop:/ protocol is set as url for the Folder Widget/View (e.g. if you select “Show the Desktop folder” or write “desktop://” in “Specify a folder:”). So as an easy workaround, you can point to your Desktop folder via the file:/ protocol in “Specify a folder:”. After I did that, the shell didn’t freeze while the “viewer” was running and the error message didn’t appear after closing it anymore.
Works too. So heads up to anyone else who gets it.
This error also occurs when you open a multimedia file from the recent documents menu. The work around given above doesn’t work for this case.