I was getting a WSOD with apache 2.4.29_1 and php-fpm 7.1.14 with the following error:
Premature end of script headers: index.php
AH01070: Error parsing script headers
This was happening on a single URL /node/add/XXX of a CT. The rest worked just fine.
I was finally able to sort it out by setting http.response.debug_cacheability_headers to FALSE on development.services.yml.
I then found out this was also mentioned here on #2827047-15: Add http.response.debug_cacheability_headers: true to development.services.yml.
According to @mpotter on #2827047-16: Add http.response.debug_cacheability_headers: true to development.services.yml:
It is actually very common on complex sites to exceed the Apache header size with too many cache tags, especially when logged in as a user looking at a listing page with lots of nodes that are cached per-user because of node-history.
My site is not that complex. It does use paragraphs and a bunch of fields.
Further research pointed me to http://httpd.apache.org/docs/2.4/mod/core.html#limitrequestfieldsize
However, attempting to set any of the following to a grater number didn't help (en either main apache conf and vhost conf)
- LimitRequestFields 100
- LimitRequestFieldSize 8190
- LimitRequestLine 8190
The values above are the default, I increased them with different values, I couldn't make it work. I am not sure if I was doing something wrong or anything but it simply didn't allow me to still access the site on that URL.
I can confirm the cache tag length was 8287 bytes on my particular use case.