Ajax issues

After updating Gutenberg on our test environment, Several in-house plugins stopped working. It seems that admin-ajax.php is too slow. The Ajax posts results in 404, while the data is successfully sent.

A cheap solution to fix this, was adding a 200 ok header in our PHP code.

But what I can’t seem to fix yet is Network XHR 500 error (PHP code resulting into data not sent) in some other forms, unless I deactivate Guttenberg.