Hello, @maxl, and welcome to the forum 
It has been reported that there’s a problem with the weaver UI for Rayya’s Children:
http://discourse.stonehearth.net/t/a15-cannot-access-weaver-build-menu/20364
and one of the devs stated that unfortunately it will be broken until the next release.
Were you playing with Ascendancy or with Rayya’s Children?