facepalm Tout bêtement 
[2022-10-21 16:24:22,977] INFO {cps.editbooks:318} converting: book id: 8319 from: EPUB to: MOBI
[2022-10-21 16:24:22,990] DEBUG {cps.services.worker:91} Add Task for user: myuser - Convert 8319
[2022-10-21 16:24:22,994] INFO {cps.tasks.convert:142} Book id 8319 - target format of .mobi does not exist. Moving forward with convert.
[2022-10-21 16:24:24,031] DEBUG {cps.tasks.convert:254} No write acces to /home/calibreweb/.config/calibre using a temporary dir instead
[2022-10-21 16:24:24,142] DEBUG {cps.tasks.convert:268} Traceback (most recent call last):
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} File "/usr/bin/ebook-convert", line 20, in <module>
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} sys.exit(main())
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} File "/usr/lib/calibre/calibre/ebooks/conversion/cli.py", line 372, in main
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} parser, plumber = create_option_parser(args, log)
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} File "/usr/lib/calibre/calibre/ebooks/conversion/cli.py", line 319, in create_option_parser
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} from calibre.ebooks.conversion.plumber import Plumber
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} File "/usr/lib/calibre/calibre/ebooks/conversion/plumber.py", line 12, in <module>
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} from calibre.customize.ui import input_profiles, output_profiles, \
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} File "/usr/lib/calibre/calibre/customize/ui.py", line 18, in <module>
[2022-10-21 16:24:24,143] DEBUG {cps.tasks.convert:268} from calibre.customize.builtins import plugins as builtin_plugins
[2022-10-21 16:24:24,144] DEBUG {cps.tasks.convert:268} File "/usr/lib/calibre/calibre/customize/builtins.py", line 752, in <module>
[2022-10-21 16:24:24,144] DEBUG {cps.tasks.convert:268} from calibre.devices.smart_device_app.driver import SMART_DEVICE_APP
[2022-10-21 16:24:24,144] DEBUG {cps.tasks.convert:268} File "/usr/lib/calibre/calibre/devices/smart_device_app/driver.py", line 2044, in <module>
[2022-10-21 16:24:24,144] DEBUG {cps.tasks.convert:268} from zeroconf import (BadTypeInNameException, _HAS_A_TO_Z,
[2022-10-21 16:24:24,144] DEBUG {cps.tasks.convert:268} ImportError: cannot import name '_HAS_A_TO_Z' from 'zeroconf' (/usr/lib/python3/dist-packages/zeroconf/__init__.py)
[2022-10-21 16:24:24,144] INFO {cps.tasks.convert:188} ebook converter failed with error while converting book
[2022-10-21 16:24:24,144] ERROR {cps.tasks.convert:192} Calibre failed with error: ImportError: cannot import name '_HAS_A_TO_Z' from 'zeroconf' (/usr/lib/python3/dist-packages/zeroconf/__init__.py)
Note : le répertoire /home/calibreweb
n’existe pas, mais apparemment ce n’est pas un soucis (utilisation d’un répertoire temporaire à la place).