Premature end of script headers: index.php (500 Error)

I’ve got Internal Server Error suddenly.
The error_log says, “Premature end of script headers: index.php” .

Only a few posts shows Internal Server Error, not the whole site.This is blog post, listing page has no error , some individual posts only.
I checked trouble shootings, and everything is okay.

server: Apache
Grav: 1.3.7
server memory: 128MB file-size: 12.9KB
cache : on
debug : on

The post has only 3 photos, each size is under 50MB, most are just text(Japanese). I prefer
html tags than markdown so it’s written in html tags.
After I’ve got the error, I reduced text almost half in volume then uploaded, with cache enable. The half size text post didn’t show 500 error, so I added rest of texts, uploaded, whole texts were shown.
But once I cleared cache then reload(F5), it shows 500 error again.

How can I show whole texts, which has a bit of long story, without 500 error? What parameter should be set with what figure?

I have googled for a couple of hours but got no idea. Would anyone help me, please?
Thank you.

If you resize or otherwise manipulate images via Gravs Asset management, PHP GD is used. Almost all Files used on the web are compressed. Manipulating them needs them to be uncompressed in Memory. If your filesize for images is larger than 10 MB, you should consider making the images smaller via an Image Program and retry.