Can confirm the problem is reproducible on mailman 3.3.0 installed via PIP.
Postorius hangs on exporting CSV subscribers [1] (~6k in this case), with error "Mailman REST API not available. Please start Mailman core" (which is otherwise running fine).
mailman3.log also shows WORKER TIMEOUT log entry, exactly the same as OP's message.
You can add a new |gunicorn.cfg| next to your |mailman.cfg| and add the additional options. For further reference if anyone reads this thread, see [2].
I had to look that up to figure out where the file name "gunicorn.cfg" came from. It's not immediately obvious to the reader why a file named "guincorn.cfg" should be specially interpreted by mailman3.
The suggested fix - increasing timeout of worker process - did work perfectly for me. Thanks :)
Regards, Daniel
[1] http://www.<redacted>.org/postorius/lists/<redacted>/csv_view/ [2] https://mailman.readthedocs.io/en/latest/src/mailman/runners/docs/rest.html#...