I ended up using the internal IPv6 address instead of localhost. I still do not understand why localhost does not work, but using the IPv6 address is fine enough in this case.

Thus, instead of

ProxyPass / http://localhost:24471/
ProxyPassReverse / http://localhost:24471/

I now use

ProxyPass / http://[fd00:0002::1]:24471/
ProxyPassReverse / http://[fd00:0002::1]:24471/

Of course, which address to use depends on your network setup.