I have noticed you are using inoremap
which binds a C-e in Insert Mode only.
I have tried your way on my VIM installation, and indeed it will type :NERDTreeToggle
in insert mode on pressing C-e.
While in normal mode there is no effect.
If it is a mistake, try changing to:
nnoremap <C-e> :NERDTreeToggle<CR>
I added it in .vimrc
- it works fine.
Also, if you still run into a problem, give another hotkey a try, you might be clashing with other binds.