ajax error when displaying a webform with address field

I get an ajax error trying to fill out a webform on a view_mode_page.
Specifically with an address field on the webform.
When selecting a country, it ajax-submits to fetch the correct address fields.
That one errors out with the cryptic "An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (@size) that this server supports."
There are no file uploads whatsoever on my webform...

The webform itself works fine when using directly through webform.
It also works fine when using it placed via a block or paragraphs on a page.
It only fails when using it on a display mode rendered through view_mode_page.

I managed to debug it to view_mode_page: MainController.php: displayEntity(): line 89:
$response = $this->httpKernel->handle($sub_request, HttpKernelInterface::SUB_REQUEST);
But I'm stuck debugging this any further...

Could someone help to get this fixed?