black screen and cursor, and nothing more

[dep]

greetings.

i'm making an attempt to switch from trinity desktop environment to 
whatever version of plasma is installed when one installs kubuntu-desktop 
in ubuntu 16.04. window manager is kwin.

when i select plasma at the login screen, all seems to go well but for the 
appearance not of plasma but of a black screen with a mouse cursor. no 
clicking of either button produces anything, and the only escape is 
ant-f1, login, and rebooting from there.

i'd guess it's a tiny configuration issue, but i have no idea what it is. 
searching the web i've found other instances similar to but not exactly 
like this one.

anybody know what's happening and how to fix it?

thanks.
-- 
dep

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