Why do the outlets in my solar-powered cottage read "open ground"?
You have faithfully built out your grounding system and kept it separate from the neutral system, which is better than a lot of people do.  It sounds like you've done this work to Code.  But what you never did is fit the neutral-ground bond in your main panel.  

A house has exactly one neutral-ground bond

And the problem is, your generator already has one of them.  

Neutral is not ground. Neutral carries the ordinary return current that the circuit needs.  Ground carries only fault current during problems.  

However, neutral is tied to ground in exactly one location.  That's a very big deal.  The point of the N-G bond is to assure that your electrical system isn't floating at thousands of volts compared to actual earth.  Because the insulation in appliances isn't rated for thousands of volts.  

But if you have two of them, that is very bad.  The ordinary current flowing on neutral will then "split" at one of the bonds, and some of it will travel over the ground wires to the other bond.  That means safety ground isn't safety ground anymore, it's a working neutral wire.  Certain failures can then cause grounds to be energized at mains voltage - which is the last thing you want.  

And the neutral-ground bond (1) is always at a grounding rod point.  You don't tie ground to earth at junction 1 then have a neutral-ground bond at junction 2.  

Right now, your neutral-ground bond only happens through the generator.  That means it's not at the same place as the grounding electrode.  That's bad.  

Put the neutral-ground bond where it belongs - in the main panel  where the grounding electrode comes in - but then you'll have two of them when you connect the generator, and that won't do.  


The cheap, but at least Code, answer is to remove the neutral-ground bonding in the generator, and then put it back when you unhook the generator to use it as a portable generator (where it needs to ground its outlets).  
The cheap, non-Code answer is to remove the ground wire from the NEMA 14 inlet where you plug in your generator.