Ah well im not expert on Nginx im afraid but there is a line in there down bottom there that gives me a hunch.
client intended to send too large body I think your PHP max body size is to low. This means if you post an image larger than that, it will fail.
Try setting these values higher in the php.ini and also thinking about giving PHP more ram to work with.
upload_max_filesize = 10M
post_max_size = 10M
client_max_body_size = 10M;
memory_limit = 64M ; Maximum amount of memory a script may consume
I find 128M works better than 64M, I work on a lot of portfolio style sites with large images.
and also I think your permissions are screwed on your temp folder (or the path is wrong):
PHP message: PHP Warning: Unknown: open(/var/lib/php/sessions/sess_utm94dsc1vgsaa5fv7788dpga7, O_RDWR) failed: Permission denied (13) in Unknown on line 0
PHP message: PHP Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php/sessions) in Unknown on line 0" while reading response header from upstream, client: 83.85.63.225, server: davlstudio.com, request: "GET /panel/ HTTP/2.0", upstream: "fastcgi://unix:/run/php/php7.0-fpm-robin.sock:", host: "davlstudio.com"
Theres some potential help on that in this thread.