Apache httpd reverse proxy returns SSL_ERROR_RX_RECORD_TOO_LONG when HTTP redirects to HTTPS

I am setting up an Apache v2.4 httpd reverse proxy for another server hosting Atlassian Confluence.

The proxy's private IP address is 10.0.0.77, its public IP address is 77.77.77.77, and a DNS A record maps the public IP to confluence.example.com.

There is a NAT in place:

• 77.77.77.77:10080 -> 10.0.0.77:80
• 77.77.77.77:10443 -> 10.0.0.77:443

which is necessary because the proxy's public IP address is used also for other services.

Name resolution on the proxy is done via /etc/hosts, which maps confluence.example.com to 10.0.0.9, the private IP of the Confluence server.

Here's /etc/httpd/conf.d/confluence.conf (as you see, it also does a redirect from HTTP to HTTPS):

<VirtualHost *:80>
    ServerName confluence.example.com
    ProxyRequests off
    ProxyPreserveHost off
    SetEnv force-proxy-request 1
    SetEnv proxy-nokeepalive 1
    ProxyPass        "/" "http://confluence.example.com:8090/" 
    ProxyPassReverse "/" "http://confluence.example.com:8090/"
    RewriteEngine On
    RewriteCond %{HTTPS} off
    RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI}
</VirtualHost>

<VirtualHost *:443>
    ServerName confluence.example.com
    ServerSignature On
        <Proxy *>
            Order deny,allow
            Allow from all
        </Proxy>
    SSLEngine on
    SSLProtocol ALL -SSLv2 -SSLv3
    SSLHonorCipherOrder on
    # SSL cipher suite shortened for clarity
    SSLCipherSuite "EECDH+ECDSA+AESGCM EECDH+aRSA+AESGCM EECDH+ECDSA+SHA384"
    SSLCertificateFile    /etc/httpd/ssl/example.crt
    SSLCertificateKeyFile /etc/httpd/ssl/example.key
    SSLCACertificateFile  /etc/httpd/ssl/example.crt
    ProxyRequests off
    ProxyPreserveHost on 
    ProxyPass        "/" "http://confluence.example.com:8090/" 
    ProxyPassReverse "/" "http://confluence.example.com:8090/"
</VirtualHost>

When accessing http://confluence.example.com:10080 (or even http://77.77.77.77:10080) from a browser, the URL changes to https://confluence.example.com:10080 but, instead of showing the Confluence login page, this error is returned:

Secure Connection Failed
An error occurred during a connection to 77.77.77.77:10080. SSL received a record that exceeded the maximum permissible length. Error code: SSL_ERROR_RX_RECORD_TOO_LONG

This is what is logged (DEBUG level) to the http access log:

33.33.33.33 - - [17/Sep/2018:17:06:59 +0200] "GET / HTTP/1.1" 302 208 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:62.0) Gecko/20100101 Firefox/62.0"
33.33.33.33 - - [17/Sep/2018:17:06:59 +0200] "\x16\x03\x01\x02" 400 226 "-" "-"

and to the http error log:

[Mon Sep 17 17:11:58.095085 2018] [core:debug] [pid 23120] protocol.c(1271): [client 33.33.33.33:49745] AH00566: request failed: malformed request line

I have set up separate https access and error logs, nothing is logged there. As you might have guessed, 33.33.33.33 is my outgoing public IP.

Accessing https://confluence.example.com:10443 works fine.

The same config works on another Apache v2.2 reverse proxy.

Any hint?