Nextcloud 18.0.4 in iframe doesn't work

When i open my nextcloud page within a iframe i receive a error message.
image

I am using nextcloud 18.0.4

I found many other articles in internet about this problem, also found many solutions, but nothing helps.

I don’t need any security, because everything is just in intranet
I also cannot understand that this is not a “normal” setting in nextloud, where i can turn on/off this feature.

Can anybody give me a hint what i can do here?