@martinmi,
Please see this Knowledge Base (KB) article. Also ensure that you have write access to the “/tmp” folder on your Linux box.
To troubleshoot the issue — try running the example python script below (lumtest.py) from the Lumerical CAD/GUI using our bundled Python3.
import lumapi
fdtd = lumapi.FDTD()
fdtd.addfdtd()
fdtd.addring()
fdtd.addmesh()
fdtd.save("mytestpy.fsp")

