1- How did you provide the switching between the generator and the solar supply ?
2-Using a sensitive meter to check for a neutral to ground lug connection can fool you as it is so sensitive a better way to check is to use some thing which draws some current like a 100 watt incandescent lamp.
3-A possible way to connect two sources which is cheap and reliable is to use two 30 non fused pullout disconnects like are used to disconnect air conditioners. Place them side by side, bring each source to the line terminals, generator to one and solar to the other. Connect the load terminals of both disconnects to the house circuit, connect the neutral form each to the house circuit neutral and the hot leg of each to the hot for the house circuit. Take one pullout and store it away use the other to connect which ever source you wish to use. It would not be possible to connect both sources at the same time.
4- Provide a medium sized stick next to the disconnects and use it to hit anyone who locates the second pull out and attempts to connect both sources at once.