iOS companion can't reach HA when connected to my home network

As the title says: As soon as I am connected to my home network via Wifi, the iOS app can’t reach Home Assistant. If I turn WIFI off it can connect to HA externally.

Internal URL: http://192.168.1.252:8123 can be accessed via PC in the same WIFI.

Any ideas? Bug in the app?

Have you added your SSID and internal url to the app settings?

It was already (automatically discovered on first app start), then I removed and added the SSID again but still not working.
Although I am able to control my light entities provided to Homekit. So, it may be an issue with the companion app itself.

This is a complete new issue to me, went from Android to iPhone.