After updating to Jenkins 2.54 (and now 2.55), the swarm client cannot connect to the server anymore when it runs on java7. When the client runs on java8, everything works fine.
Please see the log files for details.
In the log files with the log level finer (*-swarmclient-finer.log), you can see that the client behaves the same up to line 67 on both java versions.
In Jenkins 2.53, everything works as expected in both java versions.
The bug currently makes it impossible to have a slave with jdk7 running (except if you would run the client on a separately installed java8 which is quite unpractical)