As of April, 30 2020 workaround is to install a custom build of Bitbucket Branch Source plugin

Error in Jenkins logs may look like:

13:44:13  ERROR: Could not do lightweight checkout, falling back to heavyweight
13:44:13  java.io.FileNotFoundException: URL: /rest/api/1.0/projects/RTS/repos/rts-test/browse/Jenkinsfile?at=pull-requests%2F1%2Fmerge&start=0&limit=500
13:44:13       at com.cloudbees.jenkins.plugins.bitbucket.server.client.BitbucketServerAPIClient.getRequest(BitbucketServerAPIClient.java:831)
13:44:13       at com.cloudbees.jenkins.plugins.bitbucket.server.client.BitbucketServerAPIClient.getFileContent(BitbucketServerAPIClient.java:1123)
13:44:13       at com.cloudbees.jenkins.plugins.bitbucket.filesystem.BitbucketSCMFile.content(BitbucketSCMFile.java:98)
13:44:13       at jenkins.scm.api.SCMFile.contentAsString(SCMFile.java:335)
13:44:13       at org.jenkinsci.plugins.workflow.multibranch.SCMBinder.create(SCMBinder.java:107)
13:44:13       at org.jenkinsci.plugins.workflow.job.WorkflowRun.run(WorkflowRun.java:309)
13:44:13       at hudson.model.ResourceController.execute(ResourceController.java:97)
13:44:13       at hudson.model.Executor.run(Executor.java:428)
CODE

Error happens because of changes in Bitbucket 7.1 which are not supported properly by Bitbucket Branch Source plugin.

As of April, 30 2020 a workaround is to install a custom build of Bitbucket Branch Source plugin mentioned here:

https://issues.jenkins-ci.org/browse/JENKINS-61493?focusedCommentId=388268&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-388268