Lost the close file "X" in upper right corner.

The close file "X" keeps disappearing. This is not the extreme upper right close software "x", but the one that allows you to close or reduce the current file you are working on. I have uninstalled and reinstalled the X4 software a few times. The "X" reappears, but once I shut down my computer and restart, the x is no longer there. I have also reset the workspace to the factory default by holding the F8 key, this did not help at all. reinstalling the software is the only thing that seems to work for a short time. I have also tried to manipulate the toolbar tabs, but that also does not work.  It just started happening about a week ago. It makes it difficult to work on multiple files, because you cannot go to another file unless you have closed it.