Nbdev_clean fails cryptically

nbdev
13

even with clean_ids in settings.ini

I continue to have this error message:

$ nbdev_clean
/home/guillaume/miniconda/envs/dataset_tools/lib/python3.9/site-packages/nbdev/clean.py:112: UserWarning: Failed to clean notebook
  warn(f'{warn_msg}')
/home/guillaume/miniconda/envs/dataset_tools/lib/python3.9/site-packages/nbdev/clean.py:113: UserWarning: clear_all
  warn(e)
/home/guillaume/miniconda/envs/dataset_tools/lib/python3.9/site-packages/nbdev/clean.py:113: UserWarning: clear_all
  warn(e)

I use nbdev v2.3.2 but had the same behaviour with v2.2.10

fastcore             1.5.24
ghapi                1.0.3
execnb               0.1.3
nbdev                2.3.2
nbdev-numpy          0.0.292
nbdev-stdlib         0.0.309