Sending to lists does not start

Hello,
The website uses a 3rd party sending service with Mailpoet. Sending of all system emails works fine. Sending of test emails from Mailpoet works fine. However when we add a newsletter to be sent to a subscriber list it is accepted but never sends. It remains in the email list with a send count of 0/[subscriber count].

I have checked the System Status and Task Scheduler reports ‘Connection Successful’ and while there are no errors, there are some odd entries:

Cron
Accessible unknown
Status running
Last updated May 5, 2022 3:39 pm
Last run started unknown
Last run completed May 5, 2022 3:39 pm
Last seen error none
Last seen error date unknown

WP Cron / Action Scheduler Status
Version 3.5.2
Storage type DBStore
Next trigger run Invalid date
Last trigger run Invalid date
Last worker run Invalid date

Sending Queue
Status running
Started at October 19, 2022 2:56 am
Sent emails 0
Retry attempt none
Retry at none
Error none
Total completed tasks 420
Total running tasks 2
Total paused tasks 0
Total scheduled tasks 44

Is this related to the ‘Invalid date’ returned by the trigger run entries? If so which date is invalid and how can I make it valid?

Thanks