Did that upgraded to calibre 5.4.2- same error. And I still don't find any calibre.log on my file-system. But here is an excerpt from the python log file:
Spoiler:
[22.6677s] ---- stderr: ----
[22.6678s] QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-statped'
[22.6678s] Traceback (most recent call last):
[22.6678s] File "runpy.py", line 194, in _run_module_as_main
[22.6678s] File "runpy.py", line 87, in _run_code
[22.6678s] File "site.py", line 45, in <module>
[22.6678s] File "site.py", line 41, in main
[22.6678s] File "calibre/ebooks/conversion/cli.py", line 401, in main
[22.6678s] File "calibre/ebooks/conversion/plumber.py", line 1274, in run
[22.6678s] File "calibre/ebooks/conversion/plugins/docx_output.py", line 89, in convert
[22.6678s] File "calibre/ebooks/docx/writer/from_html.py", line 441, in __call__
[22.6678s] File "calibre/ebooks/docx/writer/from_html.py", line 483, in process_item
[22.6678s] File "calibre/ebooks/docx/writer/from_html.py", line 534, in process_tag
[22.6678s] File "calibre/ebooks/docx/writer/from_html.py", line 534, in process_tag
[22.6678s] File "calibre/ebooks/docx/writer/from_html.py", line 534, in process_tag
[22.6678s] [Previous line repeated 3 more times]
[22.6678s] File "calibre/ebooks/docx/writer/from_html.py", line 512, in process_tag
[22.6678s] File "calibre/ebooks/docx/writer/from_html.py", line 579, in add_block_tag
[22.6678s] File "calibre/ebooks/oeb/parse_utils.py", line 32, in barename
[22.6678s] AttributeError: 'cython_function_or_method' object has no attribute 'rpartition'
[22.6679s] -----------------
[22.6680s] En feil oppstod ved konvertering til DOCX for 861305