Internet---(wan) router (lan)-----------------(wan) pfsense (lan)---intranet
(PPPOE) ---------- 10.11.11.1 ------------- 10.11.11.2 -------- 10.12.12.1

The router lan-port has a fixed IP (10.11.11.1), pfsense wan-port has a fixed IP too (10.11.11.2).
All services work, VPN-connections work too, pfSense is set to pure-nat (advanced/misc).
Dynamic-dns update fails therefore. Its showing the actual IP in red, not green.
When ISP changes IP due to reconnect, pfsense still shows the old IP.
When i manually force an IP-check (Services/dyn-dns/edit/force update), nothing changes. Say, IP remains at old state.
If i reboot pfsense, it gets the new IP, for a short time it gets displayed in green, but then changes back to red. But its still showing the "right" new IP.

log-entry after reboot:
Mar 23 11:27:42 php-fpm 358 /rc.dyndns.update: phpDynDNS (): (Success) IP Address Updated Successfully!
Mar 23 11:27:42 php-fpm 358 /rc.dyndns.update: phpDynDNS: updating cache file /conf/dyndns_wancustom''0.cache: 81.173.152.46
Mar 23 11:27:42 php-fpm 358 /rc.dyndns.update: Dynamic DNS custom (): 81.173.152.46 extracted from Check IP Service

manual IP-check:
Mar 23 12:11:19 php-fpm 359 /services_dyndns_edit.php: Dynamic DNS (): führe get_failover_interface für wan aus. em0 gefunden
Mar 23 12:11:19 php-fpm 359 /services_dyndns_edit.php: Dynamisches DNS: updatedns() wird gestartet

from console:
curl -s https://wtfismyip.com/text -> shows actual (new) IP
same is shown from any host inside lan (pfsense-lan)