LB US iOs-devices only see black screen - no assets being loaded

For our website we have got a load balancer with geo steering.

Using an iOs-mobile device (any browser Safari or Firefox tested) requesting the website from the US we get a black screen, html is being loaded but no assets like images and css.

Doing the same test through Germany on iOs it works and the website is loading.

Requesting the website from US on Windows, Desktop in Chrome it works.

We tried purging the cache that didn’t help.

Has anybody any experience with this kind of problem?

Now I’m seeing:

Safari cannot open the page.

The error was “cannot parse response”.

With Cloudflare’s develop mode switch on / no cache the page loads without issues.