Calibre log on ubuntu

I get an error when running this command:
File "/usr/lib/python3.7/subprocess.py", line 512, in run
output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['/usr/bin/ebook-convert', '/tmp/tmpd3f2op4s/EPUB/861300.xhtml', '/tmp/tmpwbv_tgg9/861305.docx', '--chapter=/', '--chapter-mark=none', '--page-breaks-before=/', '--no-chapters-in-toc', '--toc-threshold=0', '--docx-page-size=a4', '--extra-css=/opt/produksjonssystem/xslt/nlbpub-to-docx/extra.css', '--embed-font-family=Verdana', '--docx-page-margin-top=42', '--docx-page-margin-bottom=42', '--docx-page-margin-left=70', '--docx-page-margin-right=56', '--language= (nb) ', '--base-font-size=13', '--font-size-mapping=13,13,13,13,13,13,13,13']' returned non-zero exit status 1.

This error has never occurred before and happens to only one book. Is there a log file for ebook-convert on ubuntu? I have searched the file system without finding one.

I have also tried the conversion on calibre desktop gui, and on windows the conversion runs fine.