Oh, a couple of more details.

The witness machine is also the witness for several other log shipping machines and a mirror. The messages produced are:

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DESCRIPTION:The log shipping primary database [ServerName].DBName] has backup threshold of 60 minutes and has not performed a backup log operation for 75 minutes. Check agent log and logshipping monitor information.[/font]

The job failed with this message in the SQL Agent log:

[font="Courier New"]Executed as user: [domain\ServiceAccount]. The log shipping primary database [ServerName].DBName] has backup threshold of 60 minutes and has not performed a backup log operation for 75 minutes. Check agent log and logshipping monitor information. [SQLSTATE 42000] (Error 14420). The step failed.

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Thank you in advance for your help. 🙂