Configuring a Multibranch Pipeline to use a Bitbucket Server Source fails. I know the Bitbucket Server credentials and properties are correct because I use them successfully in stand alone Pipeline builds which also use the Bitbucket Server config. Stack trace reports and authorization error in Bitbucket creating the webhook?

com.atlassian.bitbucket.jenkins.internal.client.exception.AuthorizationException: - response: 401
at com.atlassian.bitbucket.jenkins.internal.http.HttpRequestExecutorImpl.handleError(HttpRequestExecutorImpl.java:116)
at com.atlassian.bitbucket.jenkins.internal.http.HttpRequestExecutorImpl.executeRequest(HttpRequestExecutorImpl.java:81)
at com.atlassian.bitbucket.jenkins.internal.http.HttpRequestExecutorImpl.executeGet(HttpRequestExecutorImpl.java:50)
at com.atlassian.bitbucket.jenkins.internal.client.BitbucketRequestExecutor.makeGetRequest(BitbucketRequestExecutor.java:159)
at com.atlassian.bitbucket.jenkins.internal.client.BitbucketRequestExecutor.makeGetRequest(BitbucketRequestExecutor.java:87)
at com.atlassian.bitbucket.jenkins.internal.client.BitbucketWebhookClientImpl.getWebhooks(BitbucketWebhookClientImpl.java:59)
at com.atlassian.bitbucket.jenkins.internal.trigger.register.BitbucketWebhookHandler.process(BitbucketWebhookHandler.java:128)
at com.atlassian.bitbucket.jenkins.internal.trigger.register.BitbucketWebhookHandler.register(BitbucketWebhookHandler.java:54)
at com.atlassian.bitbucket.jenkins.internal.trigger.RetryingWebhookHandler.registerUsingCredentials(RetryingWebhookHandler.java:90)
at com.atlassian.bitbucket.jenkins.internal.trigger.RetryingWebhookHandler.lambda$registerWithRetry$1(RetryingWebhookHandler.java:127)
at java.util.Optional.map(Unknown Source)
at com.atlassian.bitbucket.jenkins.internal.trigger.RetryingWebhookHandler.registerWithRetry(RetryingWebhookHandler.java:126)
at com.atlassian.bitbucket.jenkins.internal.trigger.RetryingWebhookHandler.register(RetryingWebhookHandler.java:72)
Caused: com.atlassian.bitbucket.jenkins.internal.trigger.register.WebhookRegistrationFailed: Failed to register webhook in bitbucket server with url https://bitbucket.xxx.com