When a project is mirrored, If for some reason the mirror losses communication with the upstream server, and in that instance the project is deleted from the upstream server, When the mirror is reconnected to upstream, because the project no longer exist in the upstream server, a 404 error which causes full synchronization to fail, is logged.
ERROR [threadpool:thread-4] @1YWQEU9x599x248x0 10.0.1.2 "OPTIONS / HTTP/1.0" c.a.b.i.m.m.s.DefaultMirrorSynchronizationHelper [https://domain.com]#git: full synchronization failed
com.atlassian.bitbucket.internal.mirroring.mirror.UpstreamRequestFailedException: A request to the configured upstream server failed because the response was unexpected. (Status code: 404, message: '')
DEBUG [httpclient-callbacks:thread-458] @1YWQEU9x599x248x0 10.0.1.2 "OPTIONS / HTTP/1.0" c.a.b.i.m.m.c.DefaultServerUpstreamClient Failed retrieving page [start=0, limit=100] of upstream repositories for project TESTPROJECT
Completely remove and reinstall the mirror instance.