SCM Sync plugin produces error messages which don't go away when a branch of a Multibranch Pipeline get's removed:

Sat Dec 22 12:33:39 CET 2018 : Delete jobs/myFolder/jobs/myJob/branches/myBranch.fddg7f<br/>

Only way to get rid of these messages is to manually remove scm-sync-configuration.fail.log from master since the HTML code in the SCM sync status message is also not rendered.