Upgraded from 11 to 12 - using openupgrade - appears to work other than incoming email processing.

Originally found that all incoming emails where being designated to a very old crm lead "Related Document ID" (res_id) No2.

Deleted this record (crm lead Ref 2), now odoo unable to process incoming emails - with error ....

Traceback (most recent call last):
File "/opt/odoo/odoo12/addons/fetchmail/models/fetchmail.py", line 176, in fetch_mail
res_id = MailThread.with_context (** additionnal_context) .message_process (server.object_id. model, data [0] [1], save_original = server.original, strip_attachments = (not server.attach))
File "/opt/odoo/odoo12/addons/mail/models/mail_thread.py", line 1446, in message_process
routes = self.message_route (msg_txt, msg, model, thread_id, custom_values)
File "/opt/odoo/odoo12/addons/mail/models/mail_thread.py", line 1300, in message_route
update_author = True, assert_model = True, create_fallback = True)
File "/opt/odoo/odoo12/addons/mail/models/mail_thread.py", line 1057, in message_route_verify
self._routing_warn (_ ('reply to missing document (% s,% s)')% (model, thread_id), _ ('skipping'), message_id, route, assert_model)
File "/ opt / odoo / odoo12 / addons /mail/models/mail_thread.py ", line 959, in _routing_warn
raise ValueError (short_message)
ValueError: Mailbox unavailable - reply to missing document (crm.lead, None)

Any ideas please?

EDIT - As soon as I replace CRM record No 2 ... error disappears, but all incoming emails get linked to it through "record_name" and "res_id"

I've also tested by creating a new database - CRM leads works perfect here