Anyone landing here, if you are interested in trying an experimental fix, you could try the snapshot build from the PR until maintainers respond. Use Plugin Manager » Advanced.

Jesse,

Thanks for taking charge of this! I tried out your snapshot build but am still receiving the errors below. Do I still need to add these classes to the ClassFilter? The plugin reports as version "1.39.1-SNAPSHOT (private-e3afffc5-win2012-22b740$)" which I think is correct.

INFO: Jenkins is fully up and running
Jan 16, 2018 4:15:06 PM jenkins.security.ClassFilterImpl lambda$isBlacklisted$1
WARNING: org.kohsuke.github.GHPullRequestCommitDetail$Authorship in file:/var/lib/jenkins/plugins/github-api/WEB-INF/lib/github-api-1.90.jar might be dangerous, so rejecting; see https://jenkins.io/redirect/class-filter/
Jan 16, 2018 4:15:06 PM jenkins.security.ClassFilterImpl lambda$isBlacklisted$1
WARNING: org.kohsuke.github.GHUser in file:/var/lib/jenkins/plugins/github-api/WEB-INF/lib/github-api-1.90.jar might be dangerous, so rejecting; see https://jenkins.io/redirect/class-filter/
Jan 16, 2018 4:15:06 PM hudson.util.RobustReflectionConverter doUnmarshal
WARNING: Cannot convert type org.jenkinsci.plugins.ghprb.GhprbCause to type org.kohsuke.github.GHUser
Jan 16, 2018 4:15:06 PM hudson.util.RobustReflectionConverter doUnmarshal
WARNING: Cannot convert type org.jenkinsci.plugins.ghprb.GhprbCause to type org.kohsuke.github.GHUser

I was having this error: "failed to serialize hudson.model.Actionable#actions" and the update worked for me

I have to confess we don't have many jobs running with this plugin, but Daniel is right. Just kicked off a build via PR and the errors in the build.xml are gone despite the warnings in the jenkins.log.

sasquatch85 I suspect those warnings are from loading historical builds, which is not something I attempted to solve. Check whether the Jenkins /manage screen offers a Manage Old Data link. At least that would explain the first two warnings; the second two look strange to me.

Jesse,

Right on the money! Thanks a ton, I never would have guessed that. We did lose some build history for the builds between updating to 2.102 and when I installed your patched plugin but that's not a huge loss.

I probably know how to fix that (just rename the newly transient variables), but unless a plugin maintainer appears and seems ready to do a release, I am not about to spend time on retesting.

Code changed in jenkins
User: Jesse Glick
Path:
src/main/java/org/jenkinsci/plugins/ghprb/GhprbCause.java
http://jenkins-ci.org/commit/ghprb-plugin/e3afffc529941bc2674bc98f5cda3fd6944abfe8
Log:
JENKINS-48950 [JEP-200] Stop trying to serialize github-api types.

Code changed in jenkins
User: Sam Gleske
Path:
src/main/java/org/jenkinsci/plugins/ghprb/GhprbCause.java
http://jenkins-ci.org/commit/ghprb-plugin/e381590a278599f689d3394651f009faf86610cf
Log:
Merge pull request #616 from jglick/JENKINS-48950

JENKINS-48950 [JEP-200] Stop trying to serialize github-api types

Compare: https://github.com/jenkinsci/ghprb-plugin/compare/66084576abfc...e381590a2785

Fixed in GHPRB 1.40.0