It's important to use REST API to change logging level for the plugin package. The logging configuration in the Bitbucket admin area affects Bitbucket packages only

Enable debug output

You have two possibilities, activate general debug output or just for the needed classes:

General debug

To enable debug logging execute the following command:

curl -u admin:<password> -v -X PUT -H "Content-Type: application/json" <bitbucket url>/rest/api/latest/logs/logger/com.nerdwin15.stash.webhook/debug

To disable the debug output, restart your Bitbucket or run the following command:

curl -u admin:<password> -v -X PUT -H "Content-Type: application/json" <bitbucket url>/rest/api/latest/logs/logger/com.nerdwin15.stash.webhook/info

Example of logs entries for notification of Bitbucket Branch Source  plugin

[INFO] 2018-09-21 12:27:54,623 DEBUG [JenkinsWebhook:thread-1]  c.n.s.w.n.BitbucketSourceNotifier Payload is : '{"repository":{"scmId":"git","project":{"key":"PROJECT_1","name":"Project 1"},"slug":"rep_1","links":{"self":[{"href":"http://localhost:7990/bitbucket/projects/PROJECT_1/repos/rep_1/browse"}]},"public":false,"ownerName":"PROJECT_1"},"push":{"changes":[{"created":true,"closed":false,"old":null,"new":{"type":"branch","name":"feature-1","target":{"hash":"1623a82c155ac747acb335d8b236d0108f1fcd30","type":"commit"}}}]}}'

Example of logs entries for Git Plugin

[INFO] 2019-03-11 18:57:15,240 DEBUG [JenkinsWebhook:thread-1]  c.n.s.w.notifier.GitPluginNotifier Successfully triggered jenkins with url 'http://localhost:8080/jenkins/git/notifyCommit?url=http://localhost:7990/bitbucket/scm/project_1/rep_1.git&branches=master&sha1=afe80a53ddc56e6fe4a88799c379716e79c167d6':

Debugging at the Jenkins side

This plugin sends requests to Jenkins via the Jenkin Git Plugin URL http://<Jenkins URL>/git/notifyCommit?url=<SCM URL>.

Using the Jenkins logger

Jenkins has it's own logging system. For more information on gathering Jenkins logs see CloudBees instructions here.

Examining the Git Polling Log

You can also examine the Git Polling Log in the Job configuration page. The Git Plugin will only poll the repository if the URL and the branch match one of the jobs' configurations (all jobs using the Git Plugin are given the opportunity to poll if they match the request sent by Bitbucket).