What is the “Your session has expired” error message in the MS Office for the web?
The “Your session has expired” error usually happens if the Microsoft Office (Word, Excel, Powerpoint) document is opened for too long in the external editor or in some cases when using an incognito browser which clears the cookies and similar too fast.
Refresh the webpage where the document is opened.
Note: Microsoft Office for the web automatically saves your documents when changes are made, so your changes should be saved even if your session has expired.