The checkuser log table (cu_log) is now writing to the comment ID columns on all wikis (following 879946). To allow read new, previous cu_log table entries need to have their comment ID column filled.
This can be done via the PopulateCulComment maintenance script added in 879657. It looks for rows with their comment ID as 0, generates the appropriate comments and then saves them to the DB. If the generation of the comments fail for some reason, the row is skipped.