Necessity is the mother of invention
Still no response on the FreeNAS forum regarding journalctl, so, this is what I did. I deleted the Caddy log; restarted Caddy; issued the request udance.com.au/tautulli; took a snapshot of the Caddy log; isolated the lines with tautulli in them; split the (four) lines so they could be copied and this is the result.
{"level":"debug","ts":1614884735.433205,"logger":"http.handlers.reverse_proxy","msg":"upstream roundtrip","upstream":"{upstream}","request":{"remote_addr":"10.1.1.136:51911","proto":"HTTP/2.0","method":"GET","host":"udance.com.au","uri":"/tautulli","headers":{"Upgrade-Insecure-Requests"
:["1"],"Accept":["text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9"],"Sec-Fetch-User":["?1"],"Accept-Encoding":["gzip, deflate, br"],"Accept-Language":["en-GB,en-US;q=0.9,en;q=0.8"],"Cookie":["tk_or=%
22%22; tk_lr=%22%22; wordpress_logged_in_01ad53d5bf4f84d52a16c29bd439eb90=basil%7C1645606391%7Cg0BlFaRlmpOtRQIufy2y3zkZMaJQ5kBwHgodcQZTCol%7C21fc4a76a8a48a1988b5664d18c3115b251ad355646ec0b0bbf00b4cd0c0aa34; wp-settings-1=libraryContent%3Dbrowse; wp-settings-time-1=1614070400; wfwaf-auth
cookie-d3ec030b92199ac6467163e799082cd3=1%7Cadministrator%7Cb1d638a92a5d5466468d1bae6bc4744a68e7577cf00bb7bd68043c3dbc3f974d; tk_ai=woo%3AzWH9NWshxhnKTLOrnIn7Nbvx; mailchimp_landing_site=https%3A%2F%2Fudance.com.au%2Fwp-admin%2Fadmin.php%3Fpage%3Dstats%26noheader%26proxy%26chart%3Dadmin
-bar-hours-scale"],"X-Forwarded-For":["10.1.1.136"],"X-Forwarded-Proto":["https"],"User-Agent":["Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.190 Safari/537.36"],"Sec-Fetch-Site":["none"],"Sec-Fetch-Mode":["navigate"],"Sec-Fetch-Dest"
:["document"]},"tls":{"resumed":false,"version":772,"cipher_suite":4865,"proto":"h2","proto_mutual":true,"server_name":"udance.com.au"}},"duration":50.928469734,"headers":{"X-Powered-By":["PHP/7.4.15"],"Date":["Thu, 04 Mar 2021 19:05:35 GMT"],"Cache-Control":["no-cache, must-revalidate,
max-age=0"],"Content-Encoding":["gzip"],"Expires":["Wed, 11 Jan 1984 05:00:00 GMT"],"Link":["<https://udance.com.au/wp-json/>; rel=\"https://api.w.org/\""],"Server":["Caddy"],"Status":["404 Not Found"],"Content-Type":["text/html; charset=UTF-8"],"Vary":["Accept-Encoding, Cookie","Accep
t-Encoding"]},"status":404}
{"level":"error","ts":1614884735.4358041,"logger":"http.log.access.log9","msg":"handled request","request":{"remote_addr":"10.1.1.136:51911","proto":"HTTP/2.0","method":"GET","host":"udance.com.au","uri":"/tautulli","headers":{"Upgrade-Insecure-Requests":["1"],"Accept":["text/html,appli
cation/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9"],"Sec-Fetch-User":["?1"],"Accept-Encoding":["gzip, deflate, br"],"User-Agent":["Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chro
me/88.0.4324.190 Safari/537.36"],"Sec-Fetch-Site":["none"],"Sec-Fetch-Mode":["navigate"],"Sec-Fetch-Dest":["document"],"Accept-Language":["en-GB,en-US;q=0.9,en;q=0.8"],"Cookie":["tk_or=%22%22; tk_lr=%22%22; wordpress_logged_in_01ad53d5bf4f84d52a16c29bd439eb90=basil%7C1645606391%7Cg0BlFa
RlmpOtRQIufy2y3zkZMaJQ5kBwHgodcQZTCol%7C21fc4a76a8a48a1988b5664d18c3115b251ad355646ec0b0bbf00b4cd0c0aa34; wp-settings-1=libraryContent%3Dbrowse; wp-settings-time-1=1614070400; wfwaf-authcookie-d3ec030b92199ac6467163e799082cd3=1%7Cadministrator%7Cb1d638a92a5d5466468d1bae6bc4744a68e7577cf
00bb7bd68043c3dbc3f974d; tk_ai=woo%3AzWH9NWshxhnKTLOrnIn7Nbvx; mailchimp_landing_site=https%3A%2F%2Fudance.com.au%2Fwp-admin%2Fadmin.php%3Fpage%3Dstats%26noheader%26proxy%26chart%3Dadmin-bar-hours-scale"]},"tls":{"resumed":false,"version":772,"cipher_suite":4865,"proto":"h2","proto_mutu
al":true,"server_name":"udance.com.au"}},"common_log":"10.1.1.136 - - [05/Mar/2021:03:05:35 +0800] \"GET /tautulli HTTP/2.0\" 404 11073","duration":50.931240318,"size":11073,"status":404,"resp_headers":{"Status":["404 Not Found"],"Date":["Thu, 04 Mar 2021 19:05:35 GMT"],"Expires":["Wed,
11 Jan 1984 05:00:00 GMT"],"Server":["Caddy","Caddy"],"X-Powered-By":["PHP/7.4.15"],"Cache-Control":["no-cache, must-revalidate, max-age=0"],"Content-Encoding":["gzip"],"Link":["<https://udance.com.au/wp-json/>; rel=\"https://api.w.org/\""],"Content-Type":["text/html; charset=UTF-8"],"
Vary":["Accept-Encoding, Cookie","Accept-Encoding"]}}
{"level":"debug","ts":1614884741.0708818,"logger":"http.handlers.reverse_proxy","msg":"upstream roundtrip","upstream":"{upstream}","request":{"remote_addr":"10.1.1.136:51911","proto":"HTTP/2.0","method":"POST","host":"udance.com.au","uri":"/?wc-ajax=get_refreshed_fragments","headers":{"
Content-Length":["18"],"User-Agent":["Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.190 Safari/537.36"],"X-Forwarded-For":["10.1.1.136"],"X-Requested-With":["XMLHttpRequest"],"Sec-Fetch-Dest":["empty"],"Referer":["https://udance.com.au
/tautulli"],"Origin":["https://udance.com.au"],"Sec-Fetch-Mode":["cors"],"Accept-Encoding":["gzip, deflate, br"],"Cookie":["tk_or=%22%22; tk_lr=%22%22; wordpress_logged_in_01ad53d5bf4f84d52a16c29bd439eb90=basil%7C1645606391%7Cg0BlFaRlmpOtRQIufy2y3zkZMaJQ5kBwHgodcQZTCol%7C21fc4a76a8a48a1
988b5664d18c3115b251ad355646ec0b0bbf00b4cd0c0aa34; wp-settings-1=libraryContent%3Dbrowse; wp-settings-time-1=1614070400; wfwaf-authcookie-d3ec030b92199ac6467163e799082cd3=1%7Cadministrator%7Cb1d638a92a5d5466468d1bae6bc4744a68e7577cf00bb7bd68043c3dbc3f974d; tk_ai=woo%3AzWH9NWshxhnKTLOrnI
n7Nbvx; mailchimp_landing_site=https%3A%2F%2Fudance.com.au%2Fwp-admin%2Fadmin.php%3Fpage%3Dstats%26noheader%26proxy%26chart%3Dadmin-bar-hours-scale"],"Accept":["*/*"],"Content-Type":["application/x-www-form-urlencoded; charset=UTF-8"],"Sec-Fetch-Site":["same-origin"],"Accept-Language":[
"en-GB,en-US;q=0.9,en;q=0.8"],"X-Forwarded-Proto":["https"]},"tls":{"resumed":false,"version":772,"cipher_suite":4865,"proto":"h2","proto_mutual":true,"server_name":"udance.com.au"}},"duration":5.009920019,"error":"context canceled"}
{"level":"info","ts":1614884741.0712857,"logger":"http.log.access.log9","msg":"handled request","request":{"remote_addr":"10.1.1.136:51911","proto":"HTTP/2.0","method":"POST","host":"udance.com.au","uri":"/?wc-ajax=get_refreshed_fragments","headers":{"Sec-Fetch-Mode":["cors"],"Accept-En
coding":["gzip, deflate, br"],"Cookie":["tk_or=%22%22; tk_lr=%22%22; wordpress_logged_in_01ad53d5bf4f84d52a16c29bd439eb90=basil%7C1645606391%7Cg0BlFaRlmpOtRQIufy2y3zkZMaJQ5kBwHgodcQZTCol%7C21fc4a76a8a48a1988b5664d18c3115b251ad355646ec0b0bbf00b4cd0c0aa34; wp-settings-1=libraryContent%3Db
rowse; wp-settings-time-1=1614070400; wfwaf-authcookie-d3ec030b92199ac6467163e799082cd3=1%7Cadministrator%7Cb1d638a92a5d5466468d1bae6bc4744a68e7577cf00bb7bd68043c3dbc3f974d; tk_ai=woo%3AzWH9NWshxhnKTLOrnIn7Nbvx; mailchimp_landing_site=https%3A%2F%2Fudance.com.au%2Fwp-admin%2Fadmin.php%3
Fpage%3Dstats%26noheader%26proxy%26chart%3Dadmin-bar-hours-scale"],"Content-Length":["18"],"User-Agent":["Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.190 Safari/537.36"],"Origin":["https://udance.com.au"],"Sec-Fetch-Site":["same-orig
in"],"Sec-Fetch-Dest":["empty"],"Referer":["https://udance.com.au/tautulli"],"Accept-Language":["en-GB,en-US;q=0.9,en;q=0.8"],"Accept":["*/*"],"X-Requested-With":["XMLHttpRequest"],"Content-Type":["application/x-www-form-urlencoded; charset=UTF-8"]},"tls":{"resumed":false,"version":772,
"cipher_suite":4865,"proto":"h2","proto_mutual":true,"server_name":"udance.com.au"}},"common_log":"10.1.1.136 - - [05/Mar/2021:03:05:41 +0800] \"POST /?wc-ajax=get_refreshed_fragments HTTP/2.0\" 0 0","duration":5.010550976,"size":0,"status":0,"resp_headers":{"Server":["Caddy"]}}