WSQ files not opening with Pillow/wsq when using joblib.Parallel - python-3.x

Question

I am trying to preprocess large amounts of WSQ images for model training using both the Pillow and wsq libraries. To speed up my code, I am trying to use Parallel but this causes an UnidentifiedImageError.
I verified that the files are there where they should be, and that the function runs without errors when used in a regular for-loop. Other files (eg csv files) can be opened inside the function without errors, so I presume that the error lies with the combination of Parallel and Pillow/wsq. All libraries are up to date. As I am just starting out with Pillow and multiprocessing, I have no idea yet on how to fix this and any help would be highly appreciated.
Code:
from joblib import Parallel, delayed
from PIL import Image
import multiprocessing
import wsq
import numpy as np
def process_image(i):
path = "/home/user/project/wsq/image_"+str(i)+".wsq"
img = np.array(Image.open(path))
#some preprocessing, saving as npz
output_path = "/home/user/project/npz/image_"+str(i)+".npz"
np.savez_compressed(output_path, img)
return None
inputs = range(100000)
num_cores = multiprocessing.cpu_count()
Parallel(n_jobs=num_cores)(delayed(process_image)(i) for i in inputs)
Output:
joblib.externals.loky.process_executor._RemoteTraceback:
"""
Traceback (most recent call last):
File "/home/user/.local/lib/python3.8/site-packages/joblib/externals/loky/process_executor.py", line 431, in _process_worker
r = call_item()
File "/home/user/.local/lib/python3.8/site-packages/joblib/externals/loky/process_executor.py", line 285, in __call__
return self.fn(*self.args, **self.kwargs)
File "/home/user/.local/lib/python3.8/site-packages/joblib/_parallel_backends.py", line 595, in __call__
return self.func(*args, **kwargs)
File "/home/user/.local/lib/python3.8/site-packages/joblib/parallel.py", line 262, in __call__
return [func(*args, **kwargs)
File "/home/user/.local/lib/python3.8/site-packages/joblib/parallel.py", line 262, in <listcomp>
return [func(*args, **kwargs)
File "preprocess_images.py", line 9, in process_image
img = np.array(Image.open(path))
File "/home/user/.local/lib/python3.8/site-packages/PIL/Image.py", line 2967, in open
raise UnidentifiedImageError(
PIL.UnidentifiedImageError: cannot identify image file '/home/user/project/wsq/image_1.wsq'
"""
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "preprocess_images.py", line 18, in <module>
Parallel(n_jobs=num_cores)(delayed(process_image)(i) for i in inputs)
File "/home/user/.local/lib/python3.8/site-packages/joblib/parallel.py", line 1054, in __call__
self.retrieve()
File "/home/user/.local/lib/python3.8/site-packages/joblib/parallel.py", line 933, in retrieve
self._output.extend(job.get(timeout=self.timeout))
File "/home/user/.local/lib/python3.8/site-packages/joblib/_parallel_backends.py", line 542, in wrap_future_result
return future.result(timeout=timeout)
File "/usr/lib/python3.8/concurrent/futures/_base.py", line 439, in result
return self.__get_result()
File "/usr/lib/python3.8/concurrent/futures/_base.py", line 388, in __get_result
raise self._exception
PIL.UnidentifiedImageError: cannot identify image file '/home/user/project/wsq/image_1.wsq'