In my case there was no client-error with multiple building sites. The command "Game.rooms["sim"].lookForAt(FIND_CONSTRUCTION_SITES,24,27)" (the point where i have build that construction site) returned only 1 construction site too. It was also the first extension getting build, so there was no "hidden" finished building site.

My creeps try to continue building a extension in SIM. It's "Progress: 2995 / 3000" and returning "OK", so the creeps keep trying. But it is impossible to complete the building.

As there is no indication to the creeps that it's not working. So my creeps get stuck there and try again... without any success.