Jenkins 2.346.1 remoting library dependency version 4.13.2 not compatible with any swarm-plugin version.   This breakage was introduced in version 2.346.1 by this change:

•  Fix WebSocket reconnection in edge cases. Upgrade from Remoting 4.13 to 4.13.2. (pull 6576, issue 68542, Remoting 4.13.2 changelog)

The closest to compatible release of the swarm plugin is version 3.31, but that uses remoting library version 4.13.

If you pull the swarm-plugin-3.31 tag and update client/pom.xml as follows, that should fix it:

 diff --git a/client/pom.xml b/client/pom.xml
index e552bec..e8796a0 100644
--- a/client/pom.xml
+++ b/client/pom.xml
@@ -124,7 +124,7 @@
         <dependency>
             <groupId>org.jenkins-ci.main</groupId>
             <artifactId>remoting</artifactId>
-            <version>4.13</version>
+            <version>4.13.2</version>
         </dependency>
         <dependency>
             <groupId>org.apache.httpcomponents.client5</groupId>

Thanks.