Connection error: ssl_passthru:4015: not SSL (40)

Can you search for the below in your /var/log/ltm

less /var/log/ltm* | grep "SSL Handshake failed for"

This might tell you which VS its failing for. This is an info log, so by default this would be captured in your ltm log file. In the event you dont find the correct VS, then you need to increase the SSL debug logging.

tmsh modify /sys db log.ssl.level value debug