Proxy redirect not working

Configurering a domain (website) to use Proxy doesn't work in ISPConfig 3.1.13

The result of Proxy is simply a "301 Moved permanently".

I have tried proxying both HTTP to HTTP and HTTPS to HTTPS.

Apache have proxy_module and proxy_http_module:

root@isp:/# apache2ctl -M
AH00548: NameVirtualHost has no effect and will be removed in the next release /etc/apache2/sites-enabled/000-ispconfig.conf:73
Loaded Modules:
 core_module (static)
 so_module (static)
 watchdog_module (static)
 http_module (static)
 log_config_module (static)
 logio_module (static)
 version_module (static)
 unixd_module (static)
 access_compat_module (shared)
 actions_module (shared)
 alias_module (shared)
 auth_basic_module (shared)
 authn_core_module (shared)
 authn_file_module (shared)
 authz_core_module (shared)
 authz_groupfile_module (shared)
 authz_host_module (shared)
 authz_user_module (shared)
 autoindex_module (shared)
 cgi_module (shared)
 deflate_module (shared)
 dir_module (shared)
 env_module (shared)
 fastcgi_module (shared)
 fcgid_module (shared)
 filter_module (shared)
 headers_module (shared)
 include_module (shared)
 mime_module (shared)
 mpm_prefork_module (shared)
 negotiation_module (shared)
 php5_module (shared)
 proxy_module (shared)
 proxy_http_module (shared)
 python_module (shared)
 reqtimeout_module (shared)
 rewrite_module (shared)
 setenvif_module (shared)
 socache_shmcb_module (shared)
 ssl_module (shared)
 status_module (shared)
 suexec_module (shared)

Trying to Proxy from HTTP to HTTPS produces a "500 Internal Server Error" and the following entries in my error.log file:

[Sat Sep 01 16:21:06.109114 2018] [ssl:error] [pid 26731] [remote xx.xx.xx.xx:443] AH01961: SSL Proxy requested for isp.xxxxxxxx.xx:8081 but not enabled [Hint: SSLProxyEngine] [Sat Sep 01 16:21:06.109194 2018] [proxy:error] [pid 26731] AH00961: HTTPS: failed to enable ssl support for xx.xx.xx.xx:443 (ABCDEFG.org)

Related issues:

#4967 (closed) and #5115 (closed)