Problem with WATCH CALLS GRAPH

Hi all, I have a single problem in my FreePBX 2.1.3 installation:
When I look at the Reports, Daily Load, the second part of the page
should show “Hour interval” graph; If I select "FLUCTUATIONAL GRAPH"
it is OK, if I select WATCH CALLS GRAPH it display a red cross on the
left top corner.
I am using PHP5, I had to enable register_long_arrays = On on my php.ini file to make reports running.
Perhaps I am missing some required component,
but why only on this graph ? (all the others are OK !!)

thanks in advance,
Andrea