I have had this happen a couple times over the weekend, on my laptop that still won;t log in with the 'remember me' box checked.
Anyway, trying to edit a post, I could move the cursor---just no ability to actually get it to stop on the word or letter I needed to edit. For instance lets say I spelled letter---- "lette" and tried to go back and put the trailing 'r' on the word. I could not.
So I opened the full page editor and that worked---once.
Bonni had the same trouble last evening with her laptop running a different OS and was never able to actually make an edit.