Based on what you describe, you do have an unmonitored circuit – your solar. This makes perfect sense why seeing a green (generation) balance during the day, and zero balance at night.
There are a few ways your solar could be connected. If you have it backfeeding a breaker in your panel, you would put a circuit clamp on it and flag the circuit as solar. When you do this, you balance will also be zero (or thereabouts) – regardless of day/night or generation amount. You will see the exact solar generation broken out separately, along with mains and net. Alternately, if you have your solar line-tapped, you will see what you are describing since there is no clamp on the solar itself, so the software is displaying this as an unknown surplus (balance) during the day.
A photo of how you have your clamps and solar would likely allow for a better explanation, since there are several ways you have things installed.