Glass is not available for building anything

Maybe because they can only reach it via the checkpoint?

Sadly no. They can deliver the glass freely to the storage. 

Update: Somehow I can start building again. Still the glass does not show up in the resources list...

I just stumbled upon this old thread while having the same problem. Perhaps someone else googles it, so here is the solution:

 

Turns out I had selected liquid glass to display. It seems in your screenshot that you had done the same. You need to select solid glass to see the relevant value.

Solar panels simply were not available until I had manufactured enough glass to build one.