Internal Server error only on certain admin pages

Hello,

of course there is a lot of “Internal server error” topics, but I think mine is special one.

I haven’t checked the admin page of a website for a long time ( 6 months or so ) and now it gives Internal Server Error only on admin pages: Plugins, Add New Plugin, Update-core, Options-general and Themes.

I tried: disabling .htaccess, all plugins (and only one at the time too), increasing PHP memory limit by adding php.ini in wp-admin folder, reuploading core files (I had wp 4.6.1 now it is 4.6.3) and nothing helped.

I turned on Error debugging in debug.log file but it shows nothing about this internal server error (only for ex. errors when certain plugins are turned off and it can’t find path to the file etc.).

Apache server log shows:

[Tue May 23 02:02:51.092103 2017] [fastcgi:error] [pid 18253] (104)Connection reset by peer: [client xxx] FastCGI: comm with server “/usr/local/fastcgi/cgi-bin/php-cgi” aborted: read failed, referer: https://domain.com/wp-admin/edit.php?post_type=shop_order
[Tue May 23 02:02:51.092178 2017] [fastcgi:error] [pid 18253] [client xxx] FastCGI: incomplete headers (0 bytes) received from server “/usr/local/fastcgi/cgi-bin/php-cgi”, referer: https://domain.com/wp-admin/edit.php?post_type=shop_order

I am not sure if these errors are connected with this internal server error but it is quite possible.

Maybe someone has an idea what’s wrong with this? Would very appreciate the help!