Running ACY 5.10.20 on Joomla 3.9.25. The error described below was already presenting in ACY 5.10.19, but persists with the update I did this morning.
I've been running ACY 5 successfully for years, and successfully on this particular server since August 2020. Lately, I've hit a problem I can't seem to sort out. The mail server and website run on different machines. The SMTP server is a dedicated VPS with no send limits.
Lately, the send process seems to terminate unexpectedly, and I don't know why.
My configuration to send 100 emails every 6 minutes:

My external cron runs every minute, and is properly configured to trigger ACY 5 successfully.
My transaction log shows clues as to the problem, but I do not know what the clues mean:

How can I fix this?
Thanks,
Allen