Thanks, that was the problem!
You are obviously right that the path was too long. Yet this seems to neither a Windows nor a Sony specific problem since I can create that path manually without any errors. And I can open the file on the reader. Maybe Calibre is too cautious to not hitting the 260 character limit of Windows...
Is there a plugin to warn me if my template would hit the 200 character limit?
Anyway, I know what to look for. Thanks again