If you see the following error while deleting images in bulk in WordPress :
"Error 503 Service Unavailable
Varnish cache server"
Step 1. First step would be to Increase the Nginx Buffers :
Open the nginx.conf file
and update(or add if not present already) to http or server or location context :
set client body size to 2M client_max_body_size 2M;
set client body buffer size to 512K client_body_buffer_size 512K;
Step 2. Now open the nginx master file :
Add these lines just after the server block :
client_header_buffer_size 32k; large_client_header_buffers 16 512k;
Step 3. Apart from these settings, there has also been a known issue with the PHP Security Plugin. This plugins limits the maximum length allowed for a URL.
This affects bulk deleting as WordPress sends a URL with GET request with the ids of all the media to be deleted, so if someone is deleting more than 100 images at one time, the URL passed would be a very large one.
Now restart Nginx and test
service nginx restart