Nothing further happens. When I start settings from the console, then after clicking OK the console shows kf.service.services: The desktop entry file "/home/user/.local/share/solid/actions/Sync.desktop" does not have a "Type=Application" set.
What’s the problem here?
It seems the system is confused when the user wants to add an action but fails to provide any action (a command as well as various parameters) other than giving a name. 
AFAIK, such a .desktop file created as a result of a user-defined action normally includes “Type=Service”, so the message that appears in terminal might be a bug that needs fixing, probably adding more details about the real cause of failure.
The UI offers nothing but a tiny box to enter the name, so what is a user supposed to do?
Oh, this is an unexpected behavior. After defining a name, immediately a dialog box should appear to enter a command and any parameters etc. as seen below.
This is a bug to be reported then. Maybe you can try some other names that might be “acceptable” just in case?
This looks like https://bugs.kde.org/show_bug.cgi?id=518317, which should be fixed for Plasma 6.6.4.
I know, I filed that bug.
Oh, if you had mentioned that at the beginning it would be much easier to understand that you’re probably looking for some detailed information about the underlying causes of this problem, and possibly a workaround for it until it gets fixed.
I mean, I filed that bug after I created this thread 
