What do I need to do to get ProxyPass directives to register?

I am trying to make a MyCollab CE installation available via SSL, under Apache 2.4.38 (Debian). At present I am seeing ProxyPass not recognized. I have:

ProxyPass / http://localhost:8080/
ProxyPassReverse / http://localhost:8080/

When I try to load the HTTPS proxy, it gives a 500 and the error logfile is nonexistant.

My full ...le-ssl.conf file reads, mmildly sanitized:

<IfModule mod_ssl.c>
<VirtualHost *:443>
    ServerName [FQDN]
    ServerAlias [FQDN]
    DocumentRoot /home/christos/foo
    ErrorLog /var/log/apache2/foo_error.log
    CustomLog /var/log/apache2/foo_access.log combined
    ServerAdmin [email]
    <Directory />
        Options FollowSymLinks
        AllowOverride None
    </Directory>
    <Directory /home/christos/foo/>
        Options ExecCGI Indexes FollowSymLinks MultiViews
        AllowOverride None
        Order allow,deny
        allow from all
    </Directory>
    ErrorLog ${APACHE_LOG_DIR}/mycollab.error.log
    # Possible values include: debug, info, notice, warn, error, crit,
    # alert, emerg.
    LogLevel warn
    CustomLog ${APACHE_LOG_DIR}/access.log combined
    Alias /doc/ "/usr/share/doc/"
    <Directory "/usr/share/doc/">
        Options Indexes MultiViews FollowSymLinks
        AllowOverride None
        Order deny,allow
        Deny from all
        Allow from 127.0.0.0/255.0.0.0 ::1/128
    </Directory>
    ProxyPass / http://localhost:8080/
    ProxyPassReverse / http://localhost:8080/

SSLCertificateFile /etc/letsencrypt/live/[FQDN]/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/[FQDN]/privkey.pem
Include /etc/letsencrypt/options-ssl-apache.conf
</VirtualHost>
</IfModule>

I have ssl and proxy enabled.

What can I do so that visits via HTTPS turn up, under SSL, the same content as connecting directly by 8080?

--UPDATE--

I have set a LogLevel of warn. I got the following in the logs after another (single) attempt to load the homepage, and after specifying SSLEngine On and SSLProxyEngine on:

[Sun Aug 22 16:28:47.065003 2021] [socache_shmcb:debug] [pid 28984] mod_socache_shmcb.c(495): AH00831: socache_shmcb_store (0xb0 -> subcache 16)
[Sun Aug 22 16:28:47.065086 2021] [socache_shmcb:debug] [pid 28984] mod_socache_shmcb.c(849): AH00847: insert happened at idx=0, data=(0:32)
[Sun Aug 22 16:28:47.065091 2021] [socache_shmcb:debug] [pid 28984] mod_socache_shmcb.c(854): AH00848: finished insert, subcache: idx_pos/idx_used=0/1, data_pos/data_used=0/210
[Sun Aug 22 16:28:47.065094 2021] [socache_shmcb:debug] [pid 28984] mod_socache_shmcb.c(516): AH00834: leaving socache_shmcb_store successfully
[Sun Aug 22 16:28:47.067256 2021] [socache_shmcb:debug] [pid 28983] mod_socache_shmcb.c(495): AH00831: socache_shmcb_store (0x2e -> subcache 14)
[Sun Aug 22 16:28:47.067316 2021] [socache_shmcb:debug] [pid 28983] mod_socache_shmcb.c(849): AH00847: insert happened at idx=2, data=(412:444)
[Sun Aug 22 16:28:47.067321 2021] [socache_shmcb:debug] [pid 28983] mod_socache_shmcb.c(854): AH00848: finished insert, subcache: idx_pos/idx_used=0/3, data_pos/data_used=0/622
[Sun Aug 22 16:28:47.067325 2021] [socache_shmcb:debug] [pid 28983] mod_socache_shmcb.c(516): AH00834: leaving socache_shmcb_store successfully
[Sun Aug 22 16:28:47.165562 2021] [ssl:debug] [pid 28984] ssl_engine_kernel.c(383): [client 73.74.173.201:51057] AH02034: Initial (No.1) HTTPS request received for child 8 (server [FQDN]:443)
[Sun Aug 22 16:28:47.165668 2021] [authz_core:debug] [pid 28984] mod_authz_core.c(846): [client 73.74.173.201:51057] AH01628: authorization result: granted (no directives)
[Sun Aug 22 16:28:47.165717 2021] [proxy:debug] [pid 28984] mod_proxy.c(1249): [client 73.74.173.201:51057] AH01143: Running scheme http handler (attempt 0)
[Sun Aug 22 16:28:47.165723 2021] [proxy:warn] [pid 28984] [client 73.74.173.201:51057] AH01144: No protocol handler was valid for the URL / (scheme 'http'). If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.
[Sun Aug 22 16:28:47.165894 2021] [ssl:debug] [pid 28984] ssl_engine_io.c(1106): [client 73.74.173.201:51057] AH02001: Connection closed to child 8 with standard shutdown (server [FQDN]:443)
[Sun Aug 22 16:28:47.379646 2021] [ssl:debug] [pid 28983] ssl_engine_kernel.c(383): [client 73.74.173.201:51058] AH02034: Initial (No.1) HTTPS request received for child 7 (server [FQDN]:443), referer: https://[FQDN]/
[Sun Aug 22 16:28:47.379784 2021] [authz_core:debug] [pid 28983] mod_authz_core.c(846): [client 73.74.173.201:51058] AH01628: authorization result: granted (no directives), referer: https://[FQDN]/
[Sun Aug 22 16:28:47.379842 2021] [proxy:debug] [pid 28983] mod_proxy.c(1249): [client 73.74.173.201:51058] AH01143: Running scheme http handler (attempt 0), referer: https://[FQDN]/
[Sun Aug 22 16:28:47.379850 2021] [proxy:warn] [pid 28983] [client 73.74.173.201:51058] AH01144: No protocol handler was valid for the URL /favicon.ico (scheme 'http'). If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule., referer: https://[FQDN]/
[Sun Aug 22 16:28:47.380069 2021] [ssl:debug] [pid 28983] ssl_engine_io.c(1106): [client 73.74.173.201:51058] AH02001: Connection closed to child 7 with standard shutdown (server [FQDN]:443)

Try adding the below. You might not need the second one if your backend is not using SSL.

SSLEngine on

SSLProxyEngine on

If you're not getting an error log, check file permissions of the log location and ensure the user which runs apache has permission. Once you get an error log, it would be easier to troubleshoot.