If winsync is replaying a MOD operation, and the entry is not found in AD:
[datetime] NSMMReplicationPlugin - agmt="cn=agmtname" (dc1:389): windows_replay_update: Looking at modify operation local dn="uid=someuser,cn=users,cn=accounts,dc=example,dc=com" (ours,user,not group)
[datetime] NSMMReplicationPlugin - agmt="cn=agmtname" (dc1:389): map_entry_dn_outbound: looking for AD entry for DS dn="uid=someuser,cn=users,cn=accounts,dc=example,dc=com" guid="GUID"
[datetime] NSMMReplicationPlugin - Could not retrieve entry from Windows using search base [<GUID=GUID>] scope [0] filter [(objectclass=*)]: error 32:No such object
[datetime] NSMMReplicationPlugin - agmt="cn=agmtname" (dc1:389): map_entry_dn_outbound: return code -1 from search for AD entry dn="<GUID=GUID>" or dn="(null)"
[datetime] NSMMReplicationPlugin - agmt="cn=agmtname" (dc1:389): map_entry_dn_outbound: entry not found - rc -1
[datetime] NSMMReplicationPlugin - agmt="cn=agmtname" (dc1:389): windows_replay_update: Processing modify operation local dn="uid=someuser,cn=users,cn=accounts,dc=example,dc=com" remote dn="<GUID=GUID>"

winsync will attempt to proceed with the operation and will eventually call sync_acct_disable() where it will crash because ad_entry is NULL (because it could not be found)