I think I might have figured out, where the problem is. There has been incorporated new company standard families in our templates, when we upgraded from 2017 to 2018 - so you’re right, it doesn’t have anything to do with the Revit version. I can see that I can get some of the functions to work if I import a generic steel beam.
I think it might be our custom structural materials which don’t fit into the script, but I’ll have to test that out. When I run your script on one of our families, it returns an error saying that ‘material’ is not defined.
Thank you for all your help though.