Error 503 Backend unavailable

I've got the problem with SSL configuration on VPS.
Page works correctly via ip address but on the domain name i got "Error 503: Backend unavailable"

File configuration "/etc/apache2/sites-available/000-default.conf":

ServerName volante-app.pl
ProxyPreserveHost On
ProxyPass / http://54.38.53.74:8080/
ProxyPassReverse / http://54.38.53.74:8080/
ServerAlias www.volante-app.pl
DocumentRoot /var/www/html/volante-app.pl/public
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined


Options -Indexes +FollowSymlinks
AllowOverride All
Require all granted



File configuration "etc/apache2/sites-available/default-ssl.conf"


ServerName volante-app.pl:443
ServerAlias www.volante-app.pl
DocumentRoot /var/www/html/volante-app.pl/public
ErrorLog ${APACHE_LOG_DIR}/error.log
CustomLog ${APACHE_LOG_DIR}/access.log combined
SSLEngine on
SSLProxyEngine on
SSLProxyVerify none
SSLProxyCheckPeerCN off
SSLCertificateFile /etc/apache2/ssl/apache.crt
SSLCertificateKeyFile /etc/apache2/ssl/apache.key

SSLOptions +StdEnvVars


SSLOptions +StdEnvVars
DirectoryIndex index.php
AllowOverride All
Order allow,deny
Allow from all

BrowserMatch "MSIE 2-6]" \
nokeepalive ssl-unclean-shutdown \
downgrade-1.0 force-response-1.0
BrowserMatch "MSIE [17-9]" ssl-unclean-shutdown



1. I rebooted vps

2. I restarted apache

3. if I ping to domain name, I got response.

Page was working correctly via https about 2 hours but suddenly stopped (i didn't change configuration then)

on VPS is installed ubuntu 21.04

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