Issue
When browsing to K2 Workflow REST API Swagger URL, the following error is thrown:
Server Error in '/Api/Workflow/V1' Application. Runtime Error Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed.
Error Code

Resolution
After disabling customErrors (customErrors mode="Off"), more detail of the error is displayed:

In this example, the error was caused by invalid characters (invisible). Remove and re-add the above section from line 39 - 43, resolves the issue.