I wanted to delete my second post (above) as I had updated the first after (mostly) solving the issue.
Sadly both my posts got stuck waiting for a moderator. It seems the forum filters don’t like my habit of going back and editing posts more than once to get them just_right!
To work around the intermittent (non-fatal) errors I ended up adding the following lines to the end of my reverseproxy-ssl.conf file in apache:
# Avoid races (at the cost of performance) to re-use a pooled connection
# where the connection is closed
SetEnv proxy-nokeepalive 1
SetEnv force-proxy-request-1.0 1
Could you share some of your config please nachtigall?