Here are a few steps you can try to resolve this issue:

1. Check Project Consistency: Ensure that the project on your engineering station is consistent with the runtime machine. You can use the "Check Consistency" feature in PCS7 to verify this.

2. Full OS Download: Perform a full OS download again to both servers. Sometimes, a fresh download can resolve synchronization issues.

3. Server Synchronization: Make sure that both servers are properly synchronized. You can do this by:

   • Shutting down the standby OS runtime.
   • Downloading the OS into the standby server.
   • Activating the standby server and waiting for synchronization to complete.
   • Deactivating the master server.
   • Downloading the OS into the master server.
   • Activating the master server

4. Check for Manual Changes: Even though you mentioned no manual changes were made, it's worth double-checking the runtime machine for any unintended modifications.

5. Project Name and Type: Verify that the OS project name and type match exactly between the engineering station and the runtime machine.
   
   Below link also may help you

https://sieportal.siemens.com/en-WW/support/forum/posts/cannot-download-changes-to-os/303926