Strange result unit conversion on spreadsheet

[akmon]

Hello, greetings from Spain.

I was doing some conversion formulas on a spreadsheet. But There are two unexpected behaviours.
First, sometimes when I open the sheet, some cells warn with "Overflow" error, but if you edit that cell and simply press OK it fixes with the desirable result.

The other issue is the cell D6. The units are a garbage (_kN*m^2*Zm^16). The formula is a product from one unit cell to other. And if I swap the product, it gives another kind of error 9,68E335_(m^18*kN). What the...??
It´s obvious there is a bug there, I´ve attached the spreadsheet to check it by you.

Hope you find the issue. Many thanks.