The following error might occur when the application restarts (for example, at the time of application pool reset, IIS restart, and so on). In such a case, the UpdateSearchIndexObserver fails to get initialized in the constructor:
Exception type: IndexNotFoundException Exception message: Index sitecore_marketingdefinitions_web was not found at Sitecore.ContentSearch.ContentSearchManager.GetIndex(String name) at Sitecore.Marketing.Search.Observers.Activation.UpdateSearchIndexObserver`1..ctor(ILogger`1 logger, String databaseName, String searchIndex)
To resolve this issue, download and install the appropriate patch available on this page: https://github.com/SitecoreSupport/Sitecore.Support.255663/releases.
Contact Sitecore Support to get a solution for other Sitecore XP releases.