Getting 'Invalid Index' error in Projectwise Import Tool

I'm trying to import an excel sheet file using the PW import tool but during the second step I'm getting 'Invalid Index' error as shown in the attached screenshot. The import tool was working fine for so long but suddenly its not allowing me to import any further files, can anybody let me know how to resolve this issue? PW version: PW Explorer V8i (Select series 4) v08.11.11.590