Since I don’t know rails, I just replaced the website field content by an empty string. It was a spam account anyway, and it’s the only error on almost half the users already processed.
So it’s not a big issue if it is not fixed in the script code after all, though it would be nice if it didn’t block the import 
As for the attachments, I’ll try to import the messages anyway and I’ll see if attachments are imported or not (or if there are any kind of error message) if I don’t get this info in the meantime.