Since the upgrade to DocuWare 7.1 a .NET application which is using DocuWare does not work anymore.

By default, a .NET 4.6 or newer application that uses SSL to connect to another system, like DocuWare, does use TLS 1.2. So, in this case there is no need of a change. 

.NET 4.5 does support TLS 1.2, but it is not used by default and needs to be activated manually. This affects Web Services used in the DocuWare Workflow Manager, as well as REST services for Validations, but might also affect other types of 3rd party integrations.
To select TLS 1.2 as the preferred SSL protocol that .NET should use, add the code below to your implementation, before making a connection to secured resources like DocuWare Cloud:
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12 

.NET versions before .NET 4.5 does not support TLS 1.2 and therefore cannot be connected to a DocuWare 7.1 system. Please upgrade your application to a more recent version of the .NET framework.

In case you are not sure which version of .NET and therefore TLS your application is using, you can use Fiddler to identify the protocol.
The following Knowledge Base article describes how to use Fiddler, have a look at the list of "Protocols". In the given example TLS 1.0 is used: KBA-35085