HTTP error: 0 unknown error uploading in webUI

[TruffelNL]

Since I upgraded about a week ago to the latest paperless-ngx version (coming from v1.14.5) I have trouble in a very specific case. When I use my mobile phone (Samsung S22+/android) on my home WiFi and upload documents it works fine. When I use my mobile while connect to my VPN (which makes paperless available from the outside, when not connected to my WiFi) it tries to upload the files and after a minute or so, it gives the error "HTTP error: 0 unknown error". Uploading via the app also doens't work.
When I use my laptop via VPN it works fine, so it doesn't look like it's a problem with the VPN/firewall settings.
I'm not sure it's really a paperless-ngx problem, but I'm not sure what's causing it and thought I'd start here to see if someone has an idea. There are no errors in the paperless log files and no failed uploaded files in the task manager.

What can I do to start investigating this problem?

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