Intermittently builds were not triggering due to Couldn't obtain mergeable status error

[saikirankv]

The user has write access to the repo, it's happening intermittently.
May 08, 2017 11:07:38 AM SEVERE org.jenkinsci.plugins.ghprb.GhprbPullRequest checkMergeable
Couldn't obtain mergeable status.
java.io.FileNotFoundException: {"message":"Not Found","documentation_url":"https://developer.github.com/v3"}
at org.kohsuke.github.Requester.handleApiError(Requester.java:553)
at org.kohsuke.github.Requester._to(Requester.java:264)
at org.kohsuke.github.Requester.to(Requester.java:212)
at org.kohsuke.github.GHPullRequest.populate(GHPullRequest.java:205)
at org.kohsuke.github.GHPullRequest.getMergeable(GHPullRequest.java:170)
at org.jenkinsci.plugins.ghprb.GhprbPullRequest.checkMergeable(GhprbPullRequest.java:476)
at org.jenkinsci.plugins.ghprb.GhprbPullRequest.tryBuild(GhprbPullRequest.java:340)
at org.jenkinsci.plugins.ghprb.GhprbPullRequest.check(GhprbPullRequest.java:168)
at org.jenkinsci.plugins.ghprb.GhprbRepository.onPullRequestHook(GhprbRepository.java:386)
at org.jenkinsci.plugins.ghprb.GhprbTrigger.handlePR(GhprbTrigger.java:619)
at org.jenkinsci.plugins.ghprb.GhprbRootAction$2.run(GhprbRootAction.java:252)

GHPRB Plugin version= 1.33.4
Any suggestion
Thanks