This issue is reproducible on https://bioart.io/ but not locally.

There is an error in the Apache2 logs of the form:

Broken pipe: [client 41.90.70.47:1460] AH01084: pass request body failed to 127.0.0.1:5050 (127.0.0.1), referer: https://bioart.io/

It seems like the issue is the Apache2 configurations.

I tried adding the following to the VirtualHost definition, to no avail

Timeout 900
ProxyTimeout 900
KeepAliveTimeout 900
LimitRequestBody 53687091200

but it does not seem to change matters.

Adding

timeout=900

to the gunicorn configuration file fixed the issue.