[Discuss-gnuradio] GRC not responding?

[Gerard F6EEQ]

Hi,
I'm new to GNURadio (and to Linux), and I try to understand how it works.
I use the tutorials from GNU site and from ETTUS which are quite simple 
to begin.

However, I have some problems: I cannot save the flow-chart nor execute it.
When I start GRC and save the first screen (only variable + option 
squares) it's OK.
Then I enter a simple sketch (signal source + slider + throttle + FFT 
sink) and it's impossible to save or to execute.
There are no apparent error (all is black and the STOP sign is not on) 
and I counter-checked dozen of times, directly and against similar 
tutorials.

No error arise during saving and execute phases, but nothing happens.

It's probably something completely obvious, but I cannot figure what.
It worked and suddenly did not work!
Any hint would be welcome!

I use version 3.7.5 (this is the one which has been automatically 
installed with "apt-get install").
OS is the last DEBIAN with KDE desk.

Have all a nice WE.

--
73 de Gerard, F6EEQ
UFT 270, REF 27177, RCWC 257