Accursed backdrop

[Richmond]

I have just installed the Community version of LC 7.0.5 on 10 machines 
running Linux.

On one of them whenever LC is started up there is a backdrop: I am 
unable to find where,
in the preferences, I might get rid of the backdrop.

Interestingly enough all 10 of these machines have just been reformatted 
with Xubuntu 14.04.2;
half of the machines are COMPAQ Pentium 4s, the other half DELL Optiplex 
"something fairly primitive". There seems to be no obvious reason why on 
one machine alone the backdrop
should be present.

Richmond.