cvr_rasters will be a list of only raster names with no path/workspace. When you change workspace after it is created the rasters will not be found. Try adding path to each raster:
arcpy.env.workspace = CVR
cvr_rasters = arcpy.ListRasters()
cvr_rasters = [os.path.join(CVR, r) for r in cvr_rasters]
Same goes for lss_rasters since you change ...
I think to make your script work you should:
remove all the arcpy.Raster() calls
# Set names for optional output rasters, seems not working???
outDirectRad = arcpy.Raster(outNameDirect)
outDiffuseRad = arcpy.Raster(outNameDiff)
outDirectDur = arcpy.Raster(outNameDirDur)
change the call to arcpy.sa.AreaSolarRadiation like this:
# Execute ...