After changing some configurations in a vimrc, I noticed that the keys that move the cursor in Insert mode of Vim did not work. Even the Del key did nothing but move the cursor.
After examining the configuration I had added to my vimrc, I found that this behavior was caused by setting
set noesckeys. Turns out that this setting disables cursor keys in Insert mode. I did not really need that feature, so I took out that line. This leaves the default
set esckeys setting which works as expected.
Tried with: Vim 7.4 and Ubuntu 16.04