Could not load file or assembly 'HtmlAgilityPack...

Symptoms

Apple updates aren't published to SCCM.

wsus_publisher.log:
2020-03-11 10:50:38.3625|ERROR|1|Parallels.Pma.WsusPublisher.Actions.UpdatePublisher|Unable to publish product '061-72652'
System.IO.FileNotFoundException: Could not load file or assembly 'HtmlAgilityPack, Version=1.8.13.0, Culture=neutral, PublicKeyToken=bd319b19eaf3b43a' or one of its dependencies. The system cannot find the file specified.
File name: 'HtmlAgilityPack, Version=1.8.13.0, Culture=neutral, PublicKeyToken=bd319b19eaf3b43a'
   at Parallels.Pma.WsusPublisher.Core.AusProductFactory.GetPlainDescription(String base64Html)
   at Parallels.Pma.WsusPublisher.Core.AusProductFactory.ReadFrom(String id, IDictionary`2 plistData)
   at Parallels.Pma.WsusPublisher.Actions.UpdatePublisher.DoRun(FsStore fsStore, IDictionary`2 catalog, Func`2 filter)

WRN: Assembly binding logging is turned OFF.
To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.
Note: There is some performance penalty associated with assembly bind failure logging.
To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].

Cause

It's a known issue in PMM 8.1.

Resolution

Please update your PMM environment to version 8.5 or newer.

If installing the update isn't possible at this moment, as a workaround, you may install SCCM Console, PMM Console extension to the WSUS server, and reconfigure PMM Software Update Point.