When you click on any task instance of a Software Delivery task to check its status, a Warning window appears with the following message and the Task details cannot be checked:
This task run has less child instances that it should: might be partially purged.
The 'Maximum number of working database rows:' setting limit has been reached.
Follow below steps to fix this condition:
Note: Increasing these settings will allow for more storage on SQL server. Another other option to try would be to truncate the Task tables as suggested in the following KB article:
152933 Taskinstances table grows rapidly