I can confirm that Duplicati works fine behind an Apache proxy. Here’s my configuration if it’s of any use:
<VirtualHost *:80>
ServerName my.personal.subdomain
# Password protection
<Location />
AuthType Basic
AuthName "Authentication required"
AuthBasicProvider file
AuthUserFile /etc/apache2/passwords
Require user myuser
</Location>
AllowEncodedSlashes On
ProxyPass / http://localhost:8200/ disablereuse=on
ProxyPassReverse / http://localhost:8200/
</VirtualHost>