The Updater uses a message queue to determine what work it has to do. Other processes put entries on the message queue for the updater, for example, chronos puts an entry for each database on the message queue periodically. Since there is a maximum size to the message queue, it is possible that entries couldn't be added because the message queue was full.
You can change the max entrys in this message queue with the notes.ini parameter Update_Queue_Entry_Max
10000 (since version 5.0.12)
There is no kown UI setting for this notes.ini variable. But you can specify this setting in the notes.ini settings tab of the configuration settings document.