Got the same problem. This is a big major issue. I don't have administrative privileges myself (security concerns). But the game forces me to run with high rights. But Steam is not running with the Administrator account.

So this is why the error comes. How to fix it? Stop asking Windows for higher rights for this game. You really must fix that issue!

PLEASE change the manifest information (contained in the fsx.exe) from:
`<requestedExecutionLevel level="requireAdministrator" uiAccess="false"></requestedExecutionLevel>`
to
`<requestedExecutionLevel level="asInvoker" uiAccess="false"></requestedExecutionLevel>`

You might need to change the initial installation process on Steam when running the game the first time. The folder C:\ProgramData\Microsoft\FSX-SE need to be created and proper security rights must be set (write access to users). Then everything should be fine - even with normal rights.