trouble with d.mon under linux

[Andrea Bistacchi]

Dear all,
I just finished installing Grass5beta6 (intel PC with linux redhat 6.1),
but when I try opening a window I get the following error:

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                    MONITOR MENU


Making sure that the graphics monitor is running

     1 - start a graphics monitor
     2 - stop a graphics monitor

Choosing a graphics monitor for your graphics

     3 - select a graphics device for output

RETURN - quit

> 1

Enter name of graphics monitor to start
Enter "list" for a list of known monitors
Enter "list -f" for a list with current status
Hit RETURN to return to menu
> x6
can not allocate any read/write color cells
Problem selecting x6. Will try once more

hit RETURN -> 
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Obviously, after this error I cannot do anything...

I tried to follow some suggestions in the grass FAQ's, but it didn't help.

Could someone please help me?

thank you in advance

Andrea