Unable to export selected objects to .stl


I am using Silo version 2.5.4, running from Steam on my Mac. I am not able to save nor export a scene or selected objects to .stl format. This is only the case for existing .sib files. If I create a new scene, add a cube and try to export it to .stl, it works. Does anyone have any thoughts or a solution to the problem? I have also tried to make a new scene -> load into scene, and select my old .sib file, but it does not work neither.