Thanks. That gave me a helpful hint. The output was double quoted and escaped.
"trusted_proxies": [
"[\"172.18.0.3\/32\"]"
],
Wondered where that came from and found an error in my docker compose config, which lead to the proxy config from docker not being applied. Fixed that and now the config is overwritten with the docker compose config and is just the quoted IP address as expected.
The error message is gone. Thanks.