While opening the Web Client the following error occurs: "HTTP Error 503 - The service is unavailable"

Ensure that the DocuWare Application Pools in IIS are started.
If an Application Pool keeps crashing you are facing a known problem related to Microsoft Exchange Server, respectively SharePoint 2013,
please proceed according to this article: Installing Web Client on a Windows Small Business Server (SBS)
If you receive this error message after upgrading Windows, please refer to this article: HTTP Error 503 after Windows Fall Creators Update (1709)