Event ID: 602
Level: Warning
Source: NTDS ISAM
Message: NTDS (<PID>) NTDSA: Database '<drive>\NTDS\ntds.dit': Background clean-up skipped pages. The database may benefit from widening the online maintenance window during off-peak hours. If this message persists, offline defragmentation may be run to remove all skipped pages from the database.
You will see this as EventID 602 entries as below:
Online Defragmentation
repadmin /setattr <servername> "" doOnlineDefrag add 1200
This will then show this:
NTDS (840) NTDSA: Online defragmentation is beginning a full pass on database 'D:\NTDS\ntds.dit'.
Which is followed by:
NTDS (840) NTDSA: Online defragmentation has completed a full pass on database 'D:\NTDS\ntds.dit', freeing 494 pages. This pass started on 31/08/2023 and ran for a total of 12 seconds, requiring 1 invocations over 1 days. Since the database was created it has been fully defragmented 3607 times.
That will been seen in the event log as 700 and 701 Event ID's as you can see here:
NTDS (840) NTDSA: Online defragmentation is beginning a full pass on database 'D:\NTDS\ntds.dit'.
Once this command completes the defragmentation has completed and the error should cease, if you still get the error you need an offline degramentation which can be completed using this link