You are trying to configure GitLab to work with Jenkins (so the two are integrated to enable CI, continual integration, in your environment). When you test a connection of an integration webhook from GitLab to reach out to Jenkins you receive this error:
"Hook executed successfully but returned HTTP 403... No valid crumb was included in the request."
What should you do?
Possible solution #1
Warning: This is not a recommended practice. This solution will make your Jenkins system less secure.
Log into Jenkins and go to Manage Jenkins -> Configure Global Security -> CSRF Protection. Uncheck the option for "Prevent Cross Site Request Forgery exploits". Click "Save".
Possible solution #2
Use the "Build Authorization Token Root Plugin" for Jenkins. If you need directions for installing it and configuring it, see this posting (How do you troubleshoot the GitLab integration webhook error "Permission...)