So i did some further testing with the other problem we have.
We use a Lockdown screen with two Links to seperate websites. One internal and one external adress.
The external adress is configured with the prefix "Browser://" and the internal with "http://" in the Lockdown configuration.With the use of http we wanted a browser to open without the adress line, options and so on. And this works fine.

With the exception of some cases where either the external or the internal adress works. Not both of them. We tried to fix the Problem with wifi restarts and device restarts but these only have the effect that the not working adress switches between the two.

With our network team I found out that it seems like the device uses either the proxy or not. But it does not seem to use the proxy.pac file that it should use and which is configured in the WLAN settings.