So I found the solution.
Added to nginx config file here
/home/user/conf/web/nginx.conf
the following settings:
proxy_buffer_size 128k;
proxy_buffers 4 256k;
proxy_busy_buffers_size 256k;
And it started to work fine here:
http://ebrua.com/iv-admin/
Domain: ebrua.com
[not registered in a gallery]
User: angel
Host server: 80.69.77.192
login: admin
pass: admin
And it still doesn't work here:
http://ebru-art.com.ua/iv-admin/
Domain: ebrua.com
[LICENSED and REGISTERED in a gallery in your database]
User: angel (the same)
Host server: 80.69.77.192 (the same)
login: admin
pass: admin
Bothe domain and user were deleted and recreated from the very beginning.
The same settings for Apache/Nginx and all other environment for both domains used.
I also was able to get errors here:
File /var/lib/php/session/sess_qv5h36vklhibpmsvrgdibp2286:
imagevue|a:4:{s:6:"errors";a:2:{i:0;a:4:{s:8:"severity";s:6:"Notice";s:7:"message";s:22:"Undefined index: title";s:8:"filepath";s:37:"/iv-includes/include/ivFile.class.php";s:4:"line";i:41;}i:1;a:4:{s:8:"severity";s:6:"Notice";s:7:"message";s:28:"Undefined index: description";s:8:"filepath";s:37:"/iv-includes/include/ivFile.class.php";s:4:"line";i:42;}}s:5:"login";s:5:"admin";s:8:"messages";a:0:{}s:14:"lastManageLink";s:12:"?path=event/";}
And on screen:
Imagevue X2.8.10.3 Blog Facebook Twitter Feedback Dashboard Authorize
Warning: Invalid callback ivErrors::add, class 'ivErrors' not found in Unknown on line 0 Warning:
Unknown: open(/var/lib/php/session/sess_qv5h36vklhibpmsvrgdibp2286, O_RDWR)
failed: Permission denied (13) in Unknown on line 0
Warning: Invalid callback ivErrors::add, class 'ivErrors' not found in Unknown on line 0
Warning: Unknown: Failed to write session data (files).
Please verify that the current setting of session.save_path is correct (/var/lib/php/session) in Unknown on line 0
Conclusions:
If the the domain was not registered, the gallery works fine (after settings to nginx.conf file were added).
If the domain was registered the gallery doesn't work because of some errors in a registration modules.
P.S.
I din't touch any kind of owner/permissions on both domains.