Hi,
Lv 5.1 and 6 don't get along too well on the same machine.
When you have installed LV 6 it has updated the Nireports.dll, this may cause errors in LV5.1.
My suggestion is to uninstall LV6, re-install LV 5.1 and everithing should be fine.
I experienced problems like that before and I keep LV 5.1 and LV 6 on different PC's.
Good luck,
Alberto