The format of the schedule attribute of the following object is unrecognizable

Symptoms:
Knowledge Consistency Checker logs “The format of the schedule attribute of the following object is unrecognizable. ” error in Directory Service log on a Domain Controller.

Error message:
Log Name: Directory Service
Source: Microsoft-Windows-ActiveDirectory_DomainService
Event ID: 1121
Task Category: Knowledge Consistency Checker
Level: Error
Keywords: Classic
User: ANONYMOUS LOGON
Computer: LON-DC01.contoso.com
Description:
The format of the schedule attribute of the following object is unrecognizable.

Object:
CN=Lon2NYC,CN=IP,CN=Inter-Site Transports,CN=Sites,CN=Configuration,DC=contoso,DC=com

A default schedule will be substituted. This event will continue to occur until the schedule attribute on this object has been corrected.

User Action
Modify the schedule attribute.

Resolution:
The error is logged when there is no allowed period when replication can run between the sites connected by the Site Link referenced in the error, so replication fails back to the default 3 hours.
To resolve the issue open Active Directory Sites and Services snap-in, expand to Sites > Inter-site Transports > IP, then locate the Site Link highlighted in the error, and in the Properties of the Site Link on the General tab click on Change Schedule… and make sure there are period(s) when replication is allowed.

replication
The corrected replication schedule will need to replicate to sites that are connected by the Site Link, which can take up to 3 hours.