Inventor Check in Confusion

[Stemdriller]

Hi All

We have just come up with an issue that we cannot explain.

We open an Assembly in Inventor with 200 odd parts in it's structure, if we make no change what so ever and simply try and 'Check in' via Vault Tab on ribbon  we get an error, See below

However, if we simply click the X on the top right hand corner of the active screen above the navigation cube, it will allow us to check in and give us to the option to save changes to active file or any of it's dependants?

So my question really is what does X close procedure do that the 'Vault Check in' procedure does not?

GW