The first dump shows that the value of $hStep was set to the string "300" and converted to numeric (or vice versa.) My copy of RRDtool doesn't do that; only IOK is set and not POK. I'm using version 1.2.12 of RRDtool, the current is 1.2.15. You might want to check that you have a reasonably recent version of RRDtool, but as others have pointed out, that certainly isn't a direct cause of your zero modulus error.