error message when saving in excel
 

We have a shared excel file in our office. When you open it and save on it,
it will let you save once. The next time you save it gives you an error
message saying "you are no longer connected to this file...another user may
have removed you from it, or saved over it...."

Then if you close the file without saving and reopen it, it will let you
save it once and then again on the second save you will get an error message.

What's the solution?