Odoo thread times out when making pdf

I am having an issue with Odoo 15 (although tested and is happening in 14 too) where any time Odoo is asked to do a task that requires making a pdf (send an email quote, download an email quote) I just get a spinning wheel and eventually it times out.

I can't see anything in the useful in logs apart from the timeout, the timeout log doesn't show any errors just point to a whole bunch of different lines in different files.

CPU and memory are fine.

This is in a Linode instance, behind a Traefik reverse proxy it lets encrypt, but I can't imagine that would affect the pdf.

 WARNING ? odoo.service.server: Thread <Thread(odoo.service.http.request.140101450159872, started 140101450159872)> virtual real time limit (159/120s) reached.
2022-10-08 09:46:22,087 1 INFO ? odoo.service.server: Dumping stacktrace of limit exceeding threads before reloading
2022-10-08 09:46:22,097 1 INFO ? odoo.tools.misc:
# Thread: <Thread(odoo.service.http.request.140101450159872, started 140101450159872)> (db:mydb) (uid:2) (url:http://mydomain.com/web/dataset/call_kw/mail.compose.message/onchange)
File: "/usr/lib/python3.9/threading.py", line 912, in _bootstrap
  self._bootstrap_inner()
File: "/usr/lib/python3.9/threading.py", line 954, in _bootstrap_inner
  self.run()
File: "/usr/lib/python3.9/threading.py", line 892, in run
  self._target(*self._args, **self._kwargs)
File: "/usr/lib/python3.9/socketserver.py", line 650, in process_request_thread
  self.finish_request(request, client_address)
File: "/usr/lib/python3.9/socketserver.py", line 360, in finish_request
  self.RequestHandlerClass(request, client_address, self)
File: "/usr/lib/python3.9/socketserver.py", line 720, in __init__
  self.handle()
File: "/usr/lib/python3/dist-packages/werkzeug/serving.py", line 345, in handle
  BaseHTTPRequestHandler.handle(self)
File: "/usr/lib/python3.9/http/server.py", line 427, in handle
  self.handle_one_request()
File: "/usr/lib/python3/dist-packages/werkzeug/serving.py", line 379, in handle_one_request
  return self.run_wsgi()
File: "/usr/lib/python3/dist-packages/werkzeug/serving.py", line 323, in run_wsgi
  execute(self.server.app)
File: "/usr/lib/python3/dist-packages/werkzeug/serving.py", line 312, in execute
  application_iter = app(environ, start_response)
File: "/usr/lib/python3/dist-packages/odoo/service/server.py", line 482, in app
  return self.app(e, s)
File: "/usr/lib/python3/dist-packages/odoo/service/wsgi_server.py", line 112, in application
  return application_unproxied(environ, start_response)
File: "/usr/lib/python3/dist-packages/odoo/service/wsgi_server.py", line 87, in application_unproxied
  result = odoo.http.root(environ, start_response)
File: "/usr/lib/python3/dist-packages/odoo/http.py", line 1336, in __call__
  return self.dispatch(environ, start_response)
File: "/usr/lib/python3/dist-packages/odoo/http.py", line 1302, in __call__
  return self.app(environ, start_wrapped)
File: "/usr/lib/python3/dist-packages/werkzeug/middleware/shared_data.py", line 260, in __call__
  return self.app(environ, start_response)
File: "/usr/lib/python3/dist-packages/odoo/http.py", line 1531, in dispatch
  result = ir_http._dispatch()
File: "/usr/lib/python3/dist-packages/odoo/addons/website_sale/models/ir_http.py", line 15, in _dispatch
  return super(IrHttp, cls)._dispatch()
File: "/usr/lib/python3/dist-packages/odoo/addons/website/models/ir_http.py", line 198, in _dispatch
  response = super(Http, cls)._dispatch()
File: "/usr/lib/python3/dist-packages/odoo/addons/auth_signup/models/ir_http.py", line 19, in _dispatch
  return super(Http, cls)._dispatch()
File: "/usr/lib/python3/dist-packages/odoo/addons/web_editor/models/ir_http.py", line 21, in _dispatch
  return super(IrHttp, cls)._dispatch()
File: "/usr/lib/python3/dist-packages/odoo/addons/utm/models/ir_http.py", line 29, in _dispatch
  response = super(IrHttp, cls)._dispatch()
File: "/usr/lib/python3/dist-packages/odoo/addons/http_routing/models/ir_http.py", line 519, in _dispatch
  result = super(IrHttp, cls)._dispatch()
File: "/usr/lib/python3/dist-packages/odoo/addons/base/models/ir_http.py", line 237, in _dispatch
  result = request.dispatch()
File: "/usr/lib/python3/dist-packages/odoo/http.py", line 687, in dispatch
  result = self._call_function(**self.params)
File: "/usr/lib/python3/dist-packages/odoo/http.py", line 359, in _call_function
  return checked_call(self.db, *args, **kwargs)
File: "/usr/lib/python3/dist-packages/odoo/service/model.py", line 94, in wrapper
  return f(dbname, *args, **kwargs)
File: "/usr/lib/python3/dist-packages/odoo/http.py", line 348, in checked_call
  result = self.endpoint(*a, **kw)
File: "/usr/lib/python3/dist-packages/odoo/http.py", line 916, in __call__
  return self.method(*args, **kw)
File: "/usr/lib/python3/dist-packages/odoo/http.py", line 535, in response_wrap
  response = f(*args, **kw)
File: "/usr/lib/python3/dist-packages/odoo/addons/web/controllers/main.py", line 1343, in call_kw
  return self._call_kw(model, method, args, kwargs)
File: "/usr/lib/python3/dist-packages/odoo/addons/web/controllers/main.py", line 1335, in _call_kw
  return call_kw(request.env[model], method, args, kwargs)
File: "/usr/lib/python3/dist-packages/odoo/api.py", line 464, in call_kw
  result = _call_kw_multi(method, model, args, kwargs)
File: "/usr/lib/python3/dist-packages/odoo/api.py", line 451, in _call_kw_multi
  result = method(recs, *args, **kwargs)
File: "/usr/lib/python3/dist-packages/odoo/models.py", line 6475, in onchange
  record._onchange_eval(name, field_onchange[name], result)
File: "/usr/lib/python3/dist-packages/odoo/models.py", line 6208, in _onchange_eval
  method_res = method(self)
File: "/usr/lib/python3/dist-packages/odoo/addons/mail/wizard/mail_compose_message.py", line 170, in _onchange_template_id_wrapper
  values = self._onchange_template_id(self.template_id.id, self.composition_mode, self.model, self.res_id)['value']
File: "/usr/lib/python3/dist-packages/odoo/addons/mail/wizard/mail_compose_message.py", line 552, in _onchange_template_id
  values = self.generate_email_for_composer(
File: "/usr/lib/python3/dist-packages/odoo/addons/mail/wizard/mail_compose_message.py", line 663, in generate_email_for_composer
  template_values = self.env['mail.template'].with_context(tpl_partners_only=True).browse(template_id).generate_email(res_ids, fields)
File: "/usr/lib/python3/dist-packages/odoo/addons/account_edi/models/mail_template.py", line 21, in generate_email
  res = super().generate_email(res_ids, fields)
File: "/usr/lib/python3/dist-packages/odoo/addons/mail/models/mail_template.py", line 236, in generate_email
  result, format = report._render_qweb_pdf([res_id])
File: "/usr/lib/python3/dist-packages/odoo/addons/account/models/ir_actions_report.py", line 49, in _render_qweb_pdf
  return super()._render_qweb_pdf(res_ids=res_ids, data=data)
File: "/usr/lib/python3/dist-packages/odoo/addons/base/models/ir_actions_report.py", line 904, in _render_qweb_pdf
  pdf_content = self._run_wkhtmltopdf(
File: "/usr/lib/python3/dist-packages/odoo/addons/base/models/ir_actions_report.py", line 486, in _run_wkhtmltopdf
  out, err = process.communicate()
File: "/usr/lib/python3.9/subprocess.py", line 1134, in communicate
  stdout, stderr = self._communicate(input, endtime, timeout)
File: "/usr/lib/python3.9/subprocess.py", line 1981, in _communicate
  ready = selector.select(timeout)
File: "/usr/lib/python3.9/selectors.py", line 416, in select
  fd_event_list = self._selector.poll(timeout)

Start with the reported call path in addons/base/models/ir_actions_report.py, especially _run_wkhtmltopdf, and compare it with the mail template and compose-message paths shown in the trace. Reproduce PDF generation in the reported Odoo and deployment setup, then identify and document a confirmed cause and a verified fix for the timeout.