Sometimes I can't change cell contents

Hi all, I am using Office (Excel) 365. Sometimes I can't edit the cell contents. When I click on the cell and then try to edit it in the Formula Bar it won't let me. I can't even get focus in this bar. Unfortunately, this is happening often enough that it is becoming annoying. 

After some searching I came across the following post:

https://superuser.com/questions/1104286/intermittantly-cant-edit-excel-cell

This user is having exactly the same problem. I am able to edit the cell by following the indications in this post. Is this really a problem with Excel, or do I need to do something to fix this?

Thank you! Saga