Workflow Error On SharePoint : Unexpected error on server associating the workflow

Have you installed recent update for SharePoint?

When you install security update KB5002624 for SharePoint 2016, you must need to install language pack KB5002524 at the same time.

There is a new issue in this update, you might experience an issue in which SharePoint workflows can't be published because the unauthorized type is blocked. This issue also generates event tag "c42q0" in SharePoint Unified Logging System (ULS) logs.

To work around this issue, register the safe types in the Web.config file. You can look for event tag "c42q0" in ULS logs to find the blocked type. If the type and assembly are safe, add the type to the authorized list in the Web.config file.

For example:

<System.Workflow.ComponentModel.WorkflowCompiler>    
   <authorizedTypes>      
     <targetFx version="v4.0">         
       <authorizedType Assembly="Microsoft.SharePoint.WorkflowActions, Version=16.0.0.0, Culture=neutral, PublicKeyToken=null" Namespace="Microsoft.SharePoint.WorkflowActions.WithKey" TypeName="*" Authorized="True" />
   </targetFx>    
   </authorizedTypes>

Reference:

https://support.microsoft.com/en-us/topic/description-of-the-security-update-for-sharepoint-enterprise-server-2016-september-10-2024-kb5002624-db6c8b15-b9e8-4fa5-a34a-b99a0341271e

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