I have a strange problem reported by some users with various WordPress versions. I've tested this with vanilla WP 4.9.8 up to 5.1. I have no plugins activated.
Whenever I submit a
input field containing
<meta> I get a 404 error. I've recorded two videos for a better explanation (4.9.8 and 5.1)
I guess this is on a server level as a manipulation of the core files doesn't make any difference.
I'm not able to reproduce this on my end but can do this on my customer's server (see videos).
I get reports on this issue for around two weeks.