Hello,
If you try to create an additional header template we redirect you to the blog page automatically but if you don’t have a blog page then you get posts not found there. That’s not the issue because page content is not very important if you create a header structure.
I checked the debug log and found a lot of deprecated PHP notices (check the screenshot in the private area). So, either contact plugin authors to fix them or maybe try to downgrade the PHP version to 8.0 or 7.4 and reply back.
Regards