Following a (presumably quick) inspection by DA support the cause of this problem was found. There was incorrect or perhaps deprecated code in the file
/etc/httpd/conf/extra/httpd-includes.conf that was aimed at redirecting http connections to https. Incidentally this did not negatively affect squirrelmail or phpmyadmin, if only affected Roundcube.
Removing this code from this file resolved the problem.
For posterity, the code in this file (NOT WORKING in combination with the general redirect to https as outlined in the DA help text linked at the bottom) was:
Code:
################################################################################
# HTTP to HTTPS Rewrite
################################################################################
<location /phpMyAdmin>
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
</location>
<location /webmail>
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
</location>
<location /squirrelmail>
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
</location>
<location /roundcube>
RewriteEngine On
RewriteCond %{HTTPS} off
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
</location>
I can only imagine that this came from either an old DA help text or from the DA forum (since I would not generally make modifications to my DA servers that did not originate, or were described as successful, on the DA forum/website).
It probably was from some time ago since it exists in one of my DA VM templates from about a year ago (but this is basically an older template that just keeps getting updated).
Anyhow, for completeness, here is the link to forcing https in general (which I had ALSO implemented, perhaps creating a conflict/problem with the code in the
/etc/httpd/conf/extra/httpd-includes.conf) :
https://help.directadmin.com/item.php?id=624
And the link to enforcing https in Roundcube specifically:
https://help.directadmin.com/item.php?id=2082
Thanks to all who helped brainstorm on solutions and finally resolve the issue!