VPN to USG LITE 60AX behind AVM FRITZ!Box 6890 LTE not working

Lohkamp

Connected to German Telekom VDSL with a public IP port forwarding is working but no VPN access. Everything is forwarded to the USG via Exposed Host (IPv4 + IPv6), but StrongSwan always responds with received NO_PROPOSAL_CHOSEN notify error:

Jun 23 12:43:06 00[DMN] Starting IKE service (strongSwan 5.9.13, Android 14 - AP2A.240605.024/2024-06-05, Pixel 7a - google/lynx/Google, Linux 5.10.198-android13-4-00047-g7a2aa337ab82-ab11664978, aarch64, org.strongswan.android)
Jun 23 12:43:06 00[LIB] providers loaded by OpenSSL: default legacy
Jun 23 12:43:06 00[LIB] loaded plugins: androidbridge charon android-log socket-default openssl nonce pkcs1 pem x509 xcbc kdf revocation eap-identity eap-mschapv2 eap-md5 eap-gtc eap-tls
Jun 23 12:43:06 00[JOB] spawning 16 worker threads
Jun 23 12:43:06 06[IKE] initiating IKE_SA android[7] to 87.170.[... = my correct public IP]
Jun 23 12:43:06 06[ENC] generating IKE_SA_INIT request 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) N(HASH_ALG) N(REDIR_SUP) ]
Jun 23 12:43:06 06[NET] sending packet: from 10.129.[…][44570] to 87.170.[...][500] (464 bytes)
Jun 23 12:43:06 16[NET] received packet: from 87.170.[...][500] to 10.129.[…][44570] (36 bytes)
Jun 23 12:43:06 16[ENC] parsed IKE_SA_INIT response 0 [ N(NO_PROP) ]
Jun 23 12:43:06 16[IKE] received NO_PROPOSAL_CHOSEN notify error

Any idea?