Uploading via Server-Side Login via API

I wanted to ask your support in regards to uploading PDF's via a server-side login (without human intervention). We are uploading to third-party boxes, which our account has been given access to (not our own Boxes).

Error:

We are receiving the below error message when uploading the PDF's via API:

boxsdk.exception.BoxOAuthException:
Message: Grant credentials are invalid

The login to Box is valid. The error pops up when uploading to one of the folders. For example trying to upload the latest 'AU_BEL' logs the following:
Client Auth success: <boxsdk.client.client.Client object at 0x7fd2ee018310>
Client User Auth success: <boxsdk.client.client.Client object at 0x7fd2ebfd14d0>
Switched to folder for AU_BEL at 86734036620
Attempting to upload au_bel_20230821_2310.pdf to folder "86734036620"
"POST https://api.box.com/oauth2/token" 400 77
{'Date': 'Tue, 15 Aug 2023 10:18:41 GMT', 'Content-Type': 'application/json', 'Transfer-Encoding': 'chunked', 'Strict-Transport-Security': 'max-age=31536000', 'Set-Cookie': 'box_visitor_id=64db5100e92b95.63351248; expires=Thu, 15-Aug-2024 10:18:40 GMT; Max-Age=31622400; path=/; domain=.box.com; secure; SameSite=None, bv=OPS-46406; expires=Tue, 22-Aug-2023 10:18:40 GMT; Max-Age=604800; path=/; domain=.app.box.com; secure, cn=0; expires=Thu, 15-Aug-2024 10:18:40 GMT; Max-Age=31622400; path=/; domain=.app.box.com; secure, site_preference=desktop; path=/; domain=.box.com; secure', 'Cache-Control': 'no-store', 'Via': '1.1 google', 'Alt-Svc': 'h3=":443"; ma=2592000,h3-29=":443"; ma=2592000'}
{'error': 'invalid_grant', 'error_description': 'Grant credentials are invalid'}

I'm switching to the 'user' user_client (that would be me), switching to the folder, and then I get an 'invalid_grant' only when trying to upload. This seems to me that my user is somehow not allowed to upload content? However when I upload via the WebInterface, it does work. Why am I not allowed to upload via the API?

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