iCloud Add-in Keeps Disabling Itself in Outlook

you can force outlook not to disable a plugin by change loadbehavior and add it to outlook resiliency list.
It is both done by windows registry (regedit)

open regedit and look for

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\Outlook\Addins\Apple.DAV.Addin

If you can’t find it try to look for

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\Outlook\Addins\Apple.DAV.Addin

and set the value for the LoadBehavior to 3 (always load)

Even with this key set, Outlook can some times disable the plugin anyway, you can prevent it by adding another key:

HKEY_CURRENT_USER\Software\Policies\Microsoft\office\16.0\outlook\resiliency\addinlist
String: Apple.DAV.Addin
Value: 1 

HKEY_CURRENT_USER\Software\Policies\Microsoft\office\15.0\outlook\resiliency\addinlist
String: Apple.DAV.Addin
Value: 1 

HKEY_CURRENT_USER\Software\Policies\Microsoft\office\14.0\outlook\resiliency\addinlist
String: Apple.DAV.Addin
Value: 1 

Sources: https://www.slipstick.com/outlook/icloud-sync-failure/