We have a Siemens WINCC V7.1 installation with redundant server client architecture and one ES for Central Engineering.

I made some graphical changes to the HMI project in the ES. When i wanted to deploy the changes then 'Changes Only ' download option was not available.

So I gave full project download to HMI Server 1 and it worked as expected.

Next , when I gave full download to the Standby HMI server , the download process stopped with error (without any description of the error) and then I found out that the original project in standby server was also not able to open in WinCC Explorer and gave the error 'No or invalid project context. ASO cannot work correct'. 

I want to know Why the 'Changes Only' download option was not available and next What is meant by above mentioned the error and what triggered it !!!!!!

Finally I deleted the HMI project from the standby server , created a blank folder with the same name , shared the folder over network and then entire download was again done from the ES. This time download worked as expected and the standby server was put online.