I can't add more than 3 or 4 items to quotes or invoices

This problem may occurs because of a configuration of your nginx webserver. The server error should look like this:

upstream sent too big header while reading response header from upstream

To solve this problem you should try to add / update your nginx configuration with the following settings:

proxy_buffer_size   128k;
proxy_buffers   4 256k;
proxy_busy_buffers_size   256k;


fastcgi_buffer_size 128k;
fastcgi_buffers 4 256k;
fastcgi_busy_buffers_size 256k;

Please check this comment for more details.