Magento 2 Error 503 backend fetch failed

If the length of cache tags used by Magento exceed Varnish’s default of 8192 bytes, you can see HTTP 503 (Backend Fetch Failed) errors in the browser. The errors might display similar to the following:
To resolve this issue, increase the default value of the http_resp_hdr_len parameter in your Varnish configuration file. The http_resp_hdr_len parameter specifies the max header length within the total default response size of 323768 bytes.
If the http_resp_hdr_len value exceeds 32K, you must also increase the default response size using the http_resp_size parameter.

1. As a user with root privileges, open your Vanish configuration file:
CentOS 6: /etc/sysconfig/varnish
CentOS 7: /etc/varnish/varnish.params
Debian: /etc/default/varnish
Ubuntu: /etc/default/varnish

2. Edit varnish config file and search for http_resp_hdr_len parameter in this file and change this by following line:

-p http_resp_hdr_len=67000

And if you not found above parameter then add it after thread_pool_max.

See Below:

DAEMON_OPTS="-a ${VARNISH_LISTEN_ADDRESS}:${VARNISH_LISTEN_PORT} \
     -f ${VARNISH_VCL_CONF} \
     -T ${VARNISH_ADMIN_LISTEN_ADDRESS}:${VARNISH_ADMIN_LISTEN_PORT} \
     -p thread_pool_min=${VARNISH_MIN_THREADS} \
     -p thread_pool_max=${VARNISH_MAX_THREADS} \
     -p http_resp_hdr_len=67000\
     -p http_resp_size=98500 \
 -p workspace_backend=98500 \
     -S ${VARNISH_SECRET_FILE} \
     -s ${VARNISH_STORAGE}"

After this changes save file and restart server.

Hope this will helps you!