Nginx – Roundcube, upload fail

I have Roundcube installed on Ubuntu 14.04, using NGINX, with static IP, SpamAssassin, ClamAV . Sending/receiving email was fine including downloads, however cannot upload anything to send.

Configuration

php.ini file

memory_limit = 256M
max_execution_time = 60
;open_basedir =
log_errors = On
post_max_size = 100M
file_uploads = On
upload_tmp_dir = /tmp
upload_max_filesize = 100M

Folder Permissions

/tmp
srw-rw---- 1 nginx nginx

/roundcube/temp
drwxrwxrwx  2 nginx nginx   4096 Feb 20 00:41 temp

In nginx.conf, under http directive, I have set

client_max_body_size 100m;

Error log

-note: output below I removed domain, and directory names.

FastCGI sent in stderr: "PHP message: PHP Warning: File upload error – unable to create a temporary file in Unknown on line 0" while reading response header from upstream, client: X.X.X.X server: domain.com, request: "POST /mail/?_task=mail&_id=~~~from=compose&_action=upload HTTP/1.1", upstream: "fastcgi://unix:/tmp/php5-fpm.sock:", host: "domain.com", referrer: "https://domain.com/mail/?_task=mail&_action=compose&_id:~~~~

Troubleshooting

I've set the mail/temp folder to 777 just to see if it would write a file there, and it did, but Roundcube still gave the error "File Upload Failed". Normally, that directory /mail/temp is set to 755 and would write 0kb files as opposed to 777 a file actually would be written but error in roundcube remains same either way.

I also played around with the root (PHP pointed) /tmp directory permissions to see is 777 would help (all files mentioned are accessible by nginx:nginx) …no joy, same error output by Roundcube: "File Upload Failed". As a last resort I pointed PHP to the same /mail/temp to see if it would write, it did but still -same roundcube error "File Upload Failed".

How can I solved this upload problem?