in the shadow settings of that omni, set the bias wich is located in the "ray traced shadow params"-rollout to about 0,3~0,5 and try again... this should work..
O, I never use omni's with ray traced shadow... its also VERY heavy to render...
This problem is in also in 3dsmax so.. here are some piccy's:
bias=0,2
bias=0,5
bias=1
bias=13 look what it does with the shadow in the bottom near corner...
