Unable to configure the Jenkins/CI to connect with gitlab: "Expecting a valid secret token"

Issue

• There is an open issue introduced in the Jenkins's plugin: Gitlab Branch Source plugin since the 1.5.2 version affecting to the CI integration in Liferay Cloud. 
• You can find this issue if you are trying to move from another provider to Gitlab, or if you are already using a Gitlab repository and upgrades your CI to a version including a plugin's version >= 1.5.2
  
  The error in the CI logs looks like:
  For new Gitlab integrations
  

  Jul 02 15:05:58.331 build-3 [ci-0] SEVERE: Failed to initialize Jenkins
  Jul 02 15:05:58.331 build-3 [ci-0] hudson.util.HudsonFailedToLoad: org.jvnet.hudson.reactor.ReactorException: java.lang.Error: java.lang.reflect.InvocationTargetException
  	at hudson.WebAppMain$3.run(WebAppMain.java:312)
  Caused by: org.jvnet.hudson.reactor.ReactorException: java.lang.Error: java.lang.reflect.InvocationTargetException
  	at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:282)
  	at jenkins.InitReactorRunner.run(InitReactorRunner.java:49)
  	at jenkins.model.Jenkins.executeReactor(Jenkins.java:1164)
  	at jenkins.model.Jenkins.<init>(Jenkins.java:962)
  	at hudson.model.Hudson.<init>(Hudson.java:85)
  	at hudson.model.Hudson.<init>(Hudson.java:81)
  	at hudson.WebAppMain$3.run(WebAppMain.java:295)
  ....
  Caused by: java.lang.NullPointerException
  	at io.jenkins.plugins.gitlabbranchsource.GitLabHookCreator.isTokenEqual(GitLabHookCreator.java:231)
  	at io.jenkins.plugins.gitlabbranchsource.GitLabHookCreator.createWebHookWhenMissing(GitLabHookCreator.java:222)
  	at io.jenkins.plugins.gitlabbranchsource.GitLabHookCreator.register(GitLabHookCreator.java:97)
  	at io.jenkins.plugins.gitlabbranchsource.GitLabSCMSource.afterSave(GitLabSCMSource.java:731)
  	at io.jenkins.scm.GitlabSCMSourceConfiguration.getSCMSource(GitlabSCMSourceConfiguration.java:70)
  	at io.jenkins.init.SetupJob._setBranchSource(SetupJob.java:166)
  	at io.jenkins.init.SetupJob.setupJob(SetupJob.java:59)
  	... 14 more

  For existent Gitlab connections
  

  Jul 7, 11:40:05.305  build-431 [ci-0] INFO: project/main #1 completed: FAILURE
  Jul 7, 12:02:54.861  build-431 [ci-0] Jul 07, 2021 10:02:54 AM io.jenkins.plugins.gitlabbranchsource.GitLabWebHookAction isValidToken
  Jul 7, 12:02:54.861  build-431 [ci-0] WARNING: Error while validating token: null
  Jul 7, 12:02:54.866  build-431 [ci-0] Jul 07, 2021 10:02:54 AM hudson.init.impl.InstallUncaughtExceptionHandler handleException
  Jul 7, 12:02:54.866  build-431 [ci-0] WARNING: Caught unhandled exception with ID af361268-2a9c-487f-8450-223eb5837ba6
  Jul 7, 12:02:54.866  build-431 [ci-0] java.lang.Exception: Expecting a valid secret token
  	at org.kohsuke.stapler.HttpResponses.error(HttpResponses.java:91)
  	at io.jenkins.plugins.gitlabbranchsource.GitLabWebHookAction.doPost(GitLabWebHookAction.java:75)
  	at java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:627)
  ...
  	at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193)
  	at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
  	at io.jenkins.plugins.gitlabbranchsource.GitLabWebHookAction.process(GitLabWebHookAction.java:49)
  ...

Environment

• Only Liferay Cloud projects using a Gitlab repository.
• Affected CI versions:
  liferaycloud/jenkins:2.277.4-4.2.3 
liferaycloud/jenkins:2.277.4-4.2.2
  
  

Resolution

• The Liferay Cloud team is working in a solution. Meanwhile you can apply a workaround and downgrade the plugin to a working version. For this you can follow the next steps:
  
  1. Configure the CI image version liferaycloud/jenkins:2.249.3-4.2.1 in your ci/LCP.json

  2. Configure a new volume to run a clean installation setting a new label for the volume name in the ci/LCP.json:
  

   "volumes": {
     "ciDataNew": "/var/jenkins_home"
   },

  Note: You will lose your builds information in the Jenkins but you will recover the CI service. Keep in mind that the build list will continue in the Liferay Cloud's web console, and they will continue being accesible and working.
  
  3. Delete the CI service from the DXP Cloud web console, this is necessary when you are changing the volume to a "StatefulSet" service.
  
  4. Deploy the changes in the infra environment using the Command Line Tool: Run lcp deploy  from the ci/ folder and select your infra environment.
  
  

Additional Information

• Gitlab branch source issue ref
• LCP - Command Line Tool
• https://help.liferay.com/hc/en-us/articles/360061977812-Failed-Loading-plugin-after-an-update-of-the-CI-service