block[ 70143] already removed Error

I recently upgraded to Soap UI Pro 5.1.2 and i'm trying to run a test case that I've run successfully for a long time and i'm not getting this error when it's trying to load the .xls file:

"block[ 70143 ] already removed - does your POIFS have circular or duplicate block references?"

Does anyone know what would cause this and how to get around it?