Workflow Error: Failed on Start

Hi,

I'm experiencing some problems with Nintex. I created a new Nintex Workflow on a SharePoint 2013 Calendar list which only logs to Workflow History. However if i try to start the Workflow manually on a list item i get an Error. Failed on Start. Workflows got cancelled by System Account. 

The ULS says:

Legacy Workflow Infrastructure             98d8      Unexpected      Microsoft.SharePoint.SPException: <Error><CompilerError Line="0" Column="10" Text="The type or namespace name'Nintex' could not be found (are you missing a using directive or an assembly reference?)" /><CompilerError Line="0" Column="10" Text="The type or namespace name 'Nintex' could not be found (are you missing a using directive or an assembly reference?)" /><CompilerError Line="35" Column="16" Text="The type or namespace name 'Nintex' could not be found (are you missing a using directive or an assembly reference?)" /><CompilerError Line="52" Column="16" Text="The type or namespace name'Nintex' could not be found (are you missing a using directive or an assembly reference?)" /><CompilerError Line="51" Column="16" Text="The type or namespace name 'Nintex' could not be found (are you mi...

 Any ideas what is causing this issue and how to fix it?