I see two problems in your logs:

1) upstream sent too big header while reading response header from upstream

you will probably need to increase some setting in Nginx configuration, try to set:

fastcgi_buffers 16 16k;
fastcgi_buffer_size 32k;

in php handled block, usually it begins like this - location ~ \.(php|php5)$ {

2) open_basedir restriction in effect

please try to remove open_basedir restriction in php confoguration if possible

in the future we'll try to make UNA work with this restruction set - https://github.com/unaio/una/issues/1881