Why does vim say “Not Connected” when I use a remapped key to access a plugin command

Question

I'm trying to use F9 to allow NERDTree to toggle. So what I did was the following:

nnoremap <silent> <F9> :NERDTreeToggle<CR>

When I hit F9, I get the following message back from vim

Not Connected
Not Connected

Press ENTER or type a command to continue

If I do :source ~/.vimrc, my remap for F9 works as expected and NerdTree is then toggled as expected.

Also if I use the command :NERDTreeToggle at any point from command mode, it works as well.

Any ideas?

Answer 1

Using @michaelmichael's and @romainl's suggestion, I was able to determine that I had a pluging overwriting the key .

Answer is... Debug it using :verbose map and then see if you need to disable it.

Answer 2

I had the same issue - had debugger.py and debugger.vim in the ~./vim/plugin directory. I removed these unused plugins and my mappings returned to normal.

Came to light after upgrading OS X to Mavericks.