If all servers meet the system requirements, but having different versions of Windows, I would check with Qlik Support whether or not that specific case is supported.
There are big differences between Windows versions, but definitely the severity of those can be from very small (e.g.: QlikView Desktop running in Windows 7 and QlikView Server running in Windows 2012 R2) to very big (e.g.: QlikView Server running in Windows 2012 R2 and QlikView Management Service running in Windows 2008 and IIS as webserver running on Windows 2016). Even if it is supported, there could be issues between the operating systems completely alien to the QlikView deployment.