There should have been some kind of connection problem. I see you have decided that now. The next time you encounter such a situation, you should enter a field in which there are problems connecting and using telnet to find out if you can connect.
for example: from your solr window:
telnet ec2-54-247-144-120.eu-west-1.compute.amazonaws.com 2181
and then try also in zk box. He should begin to cover where your problems are.
This fixes any problems at the application level and will tell you fairly reliably, or you cannot connect. It cannot be connected, then almost always there is some kind of security problem - either the firewall (try - $ service iptables stop) works somewhere, or it is a problem with the configuration of the security group on Amazon.
The last potential problem is network availability. Despite what people think, the network is NOT reliable and should never be considered such. Anyone who works in SOA / distributed systems knows this well :) http://aphyr.com/posts/288-the-network-is-reliable
“A team from the University of Toronto and Microsoft Research studied the behavior of network failures in several Microsoft data centers. They found an average failure rate of 5.2 devices per day and 40.8 links per day with an average recovery time of approximately five minutes (and up to one week).