stretch/KDE5: icons disappeared from system tray

[solitone <solitone@mail.com>]

Hi all,

I'm using KDE 5 on stretch. Just noticed that kmix (audio mixer) and bluedevil 
(bluetooth) icons are no longer there in the system tray. Till yesterday I had 
both. 

I've found no way to restore them. I tried to remove the system tray widget 
from the panel, and added a new one (after some woes, it wasn't so 
straightforward to add it to the panel). But the new widget displays the same 
icons of the old, so no kmix nor bluedevil. 

Then I went to the system tray settings, but I don't have the entries I'm 
after, so I cannot activate them. They are not listed in the "extra items" 
section in the "general" tab, nor in the "entries" tab. 

Any idea? The only reason for the existance of kmix is to sit in the tray!