Working on more that one open Excel document
When working on 2 open Excel documents (back and forth between the two), there comes a time after 10 to 20 minutes when neither allows changes or edits any further - until one of the two documents is closed. Then the changes can be edited and saved. I can re-open the closed document and things work okay for another ten minutes or so until the problem repeats. This is a bug that Microsoft needs to address with a future update!