Some times I am getting following exception while checking isSubscribed with some topic.
Checking condition : Client.isSubscribed(topic)
Exception : com.pushtechnology.diffusion.multiplexer.MultiplexerBlockedException
This exception manifests in logs/Server.log as PUSH-000503 with description
A blocking operation failed because the multiplexers failed to process it within {} milliseconds
The default value for that timeout is 30s, which is a lifetime for a multiplexer to wait. The manual says the following: "This indicates that the server is severely overloaded or deadlocked".
On another note, version v5.5 is unsupported, and you are advised to upgrade before attempting a reproduction.