Hi,
I came across the same problem but with ArcMap 10.0. Solar Radiation tool worked at first, then it didn't for a couple of days and I tried many possible solutions before contacting the support.
The support told me to:
1. Add servicepack 5 for the ArcGIS 10.0
2. Remove background processing, doing this in ArcMap > GEOPROCESSING - GEOPROCESSING OPTIONS - and then disable BACKGROUND PROCESSES.
/Pontus