(See this comment for latest summary of this issue)

Specifying the label "windows" causes the client to go into reconnect loop as follows:

INFO: Attempting to connect to ....
May 10, 2018 1:17:30 PM hudson.plugins.swarm.SwarmClient getCsrfCrumb
SEVERE: Could not obtain CSRF crumb. Response code: 404
May 10, 2018 1:17:31 PM hudson.remoting.jnlp.Main createEngine
INFO: Setting up slave: slave-010-9533db6e
May 10, 2018 1:17:31 PM hudson.remoting.jnlp.Main$CuiListener <init>
INFO: Jenkins agent is running in headless mode.
May 10, 2018 1:17:31 PM hudson.remoting.Engine startEngine
WARNING: No Working Directory. Using the legacy JAR Cache location:...
May 10, 2018 1:17:31 PM hudson.remoting.jnlp.Main$CuiListener status
INFO: Locating server among [....]
May 10, 2018 1:17:31 PM org.jenkinsci.remoting.engine.JnlpAgentEndpointResolver resolve
INFO: Remoting server accepts the following protocols: [JNLP4-connect, CLI2-connect, JNLP-connect, Ping, CLI-connect, JNLP2-connect]
May 10, 2018 1:17:31 PM hudson.remoting.jnlp.Main$CuiListener status
INFO: Agent discovery successful
{{ Agent address: .....}}
{{ Agent port: 42442}}
{{ Identity: 2b:48:59:ed:b6:88:6b:da:b5:7c:ef:8e:f4:70:a6:41}}
May 10, 2018 1:17:31 PM hudson.remoting.jnlp.Main$CuiListener status
INFO: Handshaking
May 10, 2018 1:17:31 PM hudson.remoting.jnlp.Main$CuiListener status
INFO: Connecting to tc-jenkins-master-001.eur.ad.sag:42442
May 10, 2018 1:17:31 PM hudson.remoting.jnlp.Main$CuiListener status
INFO: Trying protocol: JNLP4-connect
May 10, 2018 1:17:31 PM hudson.remoting.jnlp.Main$CuiListener status
INFO: Remote identity confirmed: 2b:48:59:ed:b6:88:6b:da:b5:7c:ef:8e:f4:70:a6:41
May 10, 2018 1:17:31 PM hudson.remoting.jnlp.Main$CuiListener status
INFO: Connected
May 10, 2018 1:17:32 PM hudson.remoting.jnlp.Main$CuiListener status
INFO: Terminated
May 10, 2018 1:17:32 PM hudson.plugins.swarm.Client run
INFO: Retrying in 10 seconds

This occurs whether using auto-discovery or if specifying -master.  It behaves the same way with client 3.7 or 3.12.  It doesn't matter if the label is specified on command line or in a labelsFile.  There are no errors in the server log file.

The following command works: 

java -Xrs -jar swarm-client-3.7.jar -mode exclusive -name slave-010 -e
xecutors 1 -labels "test test2 windows64"

The following command fails:

java -Xrs -jar swarm-client-3.7.jar -mode exclusive -name slave-010 -e
xecutors 1 -labels "test test2 windows"

This problem did not exist in Jenkins 2.98 and swarm plugin 3.7