Could you confirm the trilead-api plugin version, Jenkins Core version, and JDK you are using? 
If you are using the latest version of trilead-api on a JDK 8 the release notes warm about is no longer compatible with Java 8.

I'm running into this same issue after updating and there isn't an option to downgrade it in the pugin manager.

okay, I was able to get this running again by manually rolling back trilead-api to 1.67.vc3938a_35172f. It looks like it's a JDK 8 issues. I thought I had until September 1st before things like this would break.

I just got burned by this same issue. Running Jenkins 2.346.2 on Java 8; now have Trilead API Plugin
Version1.71.v9e7860a_67a_df and my syslog is filling up with:

jenkins[1803]: java.lang.UnsupportedClassVersionError: com/trilead/ssh2/crypto/cipher/BlockCipher has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0 

I foolishly assumed upgrading would be safe because other plugins had notes saying they weren't updating to the latest available version, so I figured they would not install on an unsupported Java version. Shame on me for not searching for the Trilead release notes!

I'm running Jenkins 2.346.2 on JRE 11, however some agents our teams use are still running on JRE 8, and trying to run jobs on them results in this error. 

Due to Jenkins controller and Jenkins Agents share classes through the connection channel, both should run the Jenkins stuff in the same Java major version, so both should be in Java 11. 

also filed https://github.com/jenkinsci/pom/pull/297 and https://github.com/jenkinsci/trilead-ssh2/pull/107 as alternatives

Other option is to just bump jenkins.version in the plugin so that others don't accidentally get this?

Which was one of the suggestions in https://github.com/jenkins-infra/update-center2/pull/629#issue-1328437974

timja yes that would be https://github.com/jenkinsci/trilead-api-plugin/pull/55

this version is suppended from the update center so nobody can install it, the next version will run only in the next TLS that would not support Java 8.

Thank you to everyone who worked to resolve this

> this version is suppended from the update center so nobody can install it, the next version will run only in the next TLS that would not support Java 8.
 
Except anyone that is not using the UC because they may be offline, or build their own images and https://plugins.jenkins.io/trilead-api/#releases

can you please update the release notes.  "There is no guaranty that the plugin works with Java 8 anymore"  is bit miss leading here it pretty much does not work?

 Except anyone that is not using the UC because they may be offline, or build their own images and https://plugins.jenkins.io/trilead-api/#releases

It's gone from there now too.

download the `1.67.vc3938a_35172f` release `trilead-api.hpi`, then in the menu `Manager Plugins` ->`Advanced` ->`Deploy Plugin` upload this file, after restart it will be work