My working apache configuration with OH3.1
<IfModule mod_ssl.c>
<VirtualHost *:443>
ProxyPreserveHost On
ServerName openhab.mydomain.com
# Cross-Origin Resource Sharing
Header add 'Access-Control-Allow-Origin' '*'
Header add 'Access-Control-Allow_Credentials' 'true'
Header add 'Access-Control-Allow-Headers' 'Authorization,Accept,Origin,DNT,X-CustomHeader,Keep-Alive,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Content-Range,Range'
Header add 'Access-Control-Allow-Methods' 'GET,POST,OPTIONS,PUT,DELETE,PATCH'
ProxyTimeout 3600
RequestHeader unset Authorization
Header add Set-Cookie "X-OPENHAB-AUTH-HEADER=true;path=/;Secure"
<Location />
Header add Authorization ""
ProxyPass http://localhost:8080/
ProxyPassReverse http://localhost:8080/
Order allow,deny
Allow from all
AuthType Basic
AuthName "Username and Password Required"
AuthUserFile /etc/apache2/.htpasswd
Require valid-user
</Location>
SSLCertificateFile /etc/letsencrypt/live/openhab.mydomain.com/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/openhab.mydomain.com/privkey.pem
Include /etc/letsencrypt/options-ssl-apache.conf
</VirtualHost>
</IfModule>