If you see this error in your log file you will find that certain things such as the Ribbon toolbar and auto-resizing no longer work. If this happens you may also find that closing and restarting Excel does not fix the problem.
The solution is to close Excel and then look for any remaining EXCEL.EXE processes in the Windows Task Manager. To open the Task Manager right click on the Taskbar and select "Task Manager". Go to the "Details" tab and find and kill any EXCEL.EXE processes that are running.
What is happening when you see this error is that Excel has shut down, but something has prevented the EXCEL.EXE process from terminating completely. The next time you start Excel, what is actually happening is that running EXCEL.EXE process is opening a new window, but Excel is in an odd state having not been fully shutdown previously.
This can happen if an external process is preventing Excel from shutting down correctly. A common situation that can cause this is if a process is using COM and has a reference to the Excel.Application object and it has not released it. This could be a Python prompt where you've done "xl = win32com.client.Dispatch('Excel.Application')" and not released the object, or something similar indirectly using a package such as xlwings. Regardless of the cause, killing the rouge EXCEL.EXE process should solve the issue.