Fritz Box kann nur IPv4 und IPv6 getrennt.

URLs werden durch spaces getrennt:
https://dyndns.nehmer.net/update?UserName=<username>&Password=<passwd>&IPv4=<ipaddr> https://dyndns.nehmer.net/update?UserName=<username>&Password=<passwd>&IPv6=<ip6addr>&IPv6Net=<ip6lanprefix>

ergibt:

79.192.127.0 - - [22/Sep/2021:18:16:17 +0200] "GET /update?UserName=atlantis&Password=xxx&IPv4=79.192.127.157 HTTP/1.1" 500 5641 "-" "Fritz!Box DDNS/1.0.2"
79.192.127.0 - - [22/Sep/2021:18:16:18 +0200] "GET /update?UserName=atlantis&Password=xxx&IPv6=2003:ef:8fff:1021:7eff:4dff:fe7e:e901&IPv6Net=2003:ef:8f0f:fc00::/64 HTTP/1.1" 500 5621 "-" "Fritz!Box DDNS/1.0.2"

/update?UserName=atlantis&Password=xxx&IPv4=79.192.127.157
/update?UserName=atlantis&Password=xx&IPv6=2003:ef:8fff:1021:7eff:4dff:fe7e:e901&IPv6Net=2003:ef:8f0f:fc00::/64

/update?UserName=example&Password=kennwort&IPv4=79.192.127.157
/update?UserName=example&Password=kennwort&IPv6=2003:ef:8fff:1021:7eff:4dff:fe7e:e901&IPv6Net=2003:ef:8f0f:fc00::/64

/update?UserName=test&Password=kennwort&Domain=atlantis.nehmer.net&IPv4=79.202.5.79&IPv6=2003:ef:8fff:905:7eff:4dff:fe7e:e901&DualStack=1&IPv6Net=2003:ef:8f08:f600::/64

Zerlegt:
/update?
  UserName=test
  &Password=kennwort
  &Domain=atlantis.nehmer.net
  &IPv4=79.202.5.79
  &IPv6=2003:ef:8fff:905:7eff:4dff:fe7e:e901
  &DualStack=1
  &IPv6Net=2003:ef:8f08:f600::/64


ispconfig.vhost
  ProxyPass /hello http://127.0.0.1:8080/hello
  ProxyPass /update http://127.0.0.1:8080/update

http://localhost:8080/update?UserName=example&Password=kennwort&Domain=atlantis.nehmer.net&IPv4=79.202.5.79&IPv6=2003:ef:8fff:905:7eff:4dff:fe7e:e901&DualStack=1&IPv6Net=2003:ef:8f08:f600::/64

http://localhost:8080/update?UserName=example&Password=kennwort&Domain=atlantis.nehmer.net&IPv6=2003:ef:8fff:905:7eff:4dff:fe7e:e901&DualStack=1&IPv6Net=2003:ef:8f08:f600::/64

http://localhost:8080/update?UserName=example&Password=kennwort&Domain=atlantis.nehmer.net&IPv4=79.202.5.79