Webhooks are likely impacted by the same problem as is described here: https://jira.atlassian.com/browse/BSERV-11796
BSERV-11796, all that was done was to disable TLSv1.3 in the HTTP Client, however that client instance is specific to the repository importer.
The above is suspected to relate to the following JDK bug:
- JDK-8214418 HttpClient falls in running with 100% cpu usage after an error signalled on channel
1) Run Bitbucket Server with Java 8; or
2) (Not yet verified) The issue is observed with Java 11 that uses TLS v1.3. It is likely that TLS v1.2 works fine and will not cause this issue. This can be done by setting the following JVM parameter: -Dhttps.protocols=TLSv1.2 while starting bitbucket server up.