Steps to reproduce:

• Create a Bitbucket Server instance in Jenkins, and register with an invalid personal access token (either completely fake, without sufficient permissions, or one that was valid but is later revoked)
• Create a new Multibranch Pipeline job
• Configure a Bitbucket SCM Source using the server instance created previously. Make sure the job credentials are read-only (and thus cannot push a webhook)
• Check the `Bitbucket Server trigger scan after push` trigger
• Save the job

Expected Behaviour:
Jenkins informs you in a gentle and polite way that the admin token does not have webhook write permission, and to please contact your system administrator to resolve the issue.

Actual Behaviour:
Angry jenkins and a non-descriptive "401" buried in the stack trace.

Workaround
None needed- this is more a usability issue. If encountering this screen, check your token credentials have access to the resource specified in failing job.