Vim freezes after pressing CTRL-U in insert mode. Any suggestions?

Recently I figured out that pressing CTRL-U locks/freezes vim in insert mode. The only CTRL-C helps to get it back. Terminal version and gvim have the same behaviour. Only in virtual console mode (CTRL-ALT-F1) the C-U works properly. Here is my .vimrc:

" This must be first, because it changes other options as a side effect. set nocompatible " Don't use Ex mode, use Q for formatting map Q gq " CTRL-U in insert mode deletes a lot. Use CTRL-G u to first break undo, " so that you can undo CTRL-U after inserting a line break. imap <C-U> <C-G>u<C-U> " In many terminal emulators the mouse works just fine, thus enable it. if has('mouse') set mouse=a endif " Switch syntax highlighting on, when the terminal has colors " Also switch on highlighting the last used search pattern. if &t_Co > 2 || has("gui_running") syntax on set hlsearch endif " Only do this part when compiled with support for autocommands. if has("autocmd") " Enable file type detection. " Use the default filetype settings, so that mail gets 'tw' set to 72, " 'cindent' is on in C files, etc. " Also load indent files, to automatically do language-dependent indenting. filetype plugin indent on " Put these in an autocmd group, so that we can delete them easily. augroup vimrcEx au! " For all text files set 'textwidth' to 78 characters. autocmd FileType text setlocal textwidth=78 " When editing a file, always jump to the last known cursor position. " Don't do it when the position is invalid or when inside an event handler " (happens when dropping a file on gvim). " Also don't do it when the mark is in the first line, that is the default " position when opening a file. autocmd BufReadPost * \ if line("'\"") > 1 && line("'\"") <= line("$") | \ exe "normal! g`\"" | \ endif augroup END else set autoindent " always set autoindenting on endif " Convenient command to see the difference between the current buffer and the " file it was loaded from, thus the changes you made. " Only define it when not defined already. if !exists(":DiffOrig") command DiffOrig vert new | set bt=nofile | r ++edit # | 0d_ | diffthis \ | wincmd p | diffthis endif " Options " ========================================================= colorscheme gruvbox let g:gruvbox_contrast_dark = 'normal' set background=dark set t_Co=256 " spelling set spell spelllang=ru_yo,en_us set spellsuggest=10 set keymap=russian-jcukenwin set iminsert=0 set imsearch=0 " backup set writebackup " backup on write set nobackup " keep a backup file set undofile " keep an undo file set swapfile " keep a swap file " directories " set autochdir set backupdir=~/.vim/tmp/backup// set undodir=~/.vim/tmp/undo// set directory=~/.vim/tmp/swp// " active editor set nowrap set nu set backspace=indent,eol,start " backspacing over everything in im set ruler " show the cursor position all the time " cmd set history=50 " keep 50 lines of command line history set showcmd " display incomplete commands " search set ignorecase " ignore case using a search pattern. set smartcase " override 'ignorecase' when pattern has upper case. set incsearch " do incremental searching " tab " set tabstop=2 shiftwidth=2 noexpandtab " n-column tab set tabstop=2 shiftwidth=2 expandtab " n-space character tab " window set splitbelow set splitright set winminheight=0 " folding settings set foldmethod=indent " fold based on indent set foldnestmax=10 " deepest fold is 10 levels set nofoldenable " dont fold by default set foldlevel=1 " this is just what i use " invisible chars set list " don't show invisible characters. set listchars=tab:.\ ,trail:~,extends:>,precedes:<,eol:\ "¬ set nohidden " unload buffer when no longer shown in window. set laststatus=2 " Display vim-lightline " indent, cursor, width set cursorline " Highlight the screen line of the cursor. set colorcolumn=72 " Columan to highlight. set autoindent " Automatically set the indent of a new line. set textwidth=80 set completeopt=menuone,longest ",preview prevents immediate complete " tab set switchbuf=usetab " use window or tabs to find selected buff " windows set noea " fix window size after closing others " sessions (^blank,help) set sessionoptions=buffers,curdir,folds,help,options,tabpages,winsize " Autocmd " ========================================================= au BufReadPost *.tmpl set syntax=html " Mappings " ========================================================= " source and update file nmap <F5> :so ~/.vimrc<CR> nmap <leader>e :e<CR> " copy filepath nmap <silent> <F4> :let @+=expand("%:p")<CR> " buffers nmap <F2> :ls<CR>:b nmap <c-w><c-t> :tabnew<CR> nmap <c-w><c-e> :tab sp<CR> nmap <c-w><c-b>n :tabnew %<Bar>:bNext<Bar><CR> nmap <c-w><c-b><c-b> :bw %<CR> nmap <c-w><c-b>w :bw! %<CR> nmap <c-w><c-b>a :tab ball<CR> " sessions nmap <silent> <leader>ss :wa<Bar>exe "mksession! " . v:this_session<CR> nmap <leader>sr :so .session.vim<CR> nmap <leader>sc :msk .session.vim<CR> " quit nmap <leader>qd :qa!<CR> " quit & discard changes nmap <leader>qq :xa<CR> " quit & write if changed nmap <leader>qs <leader>s<leader>qd " new vertical window nmap <c-w><c-n> :vnew<CR> " tab movement and switching nmap <c-l> :tabnext<CR> nmap <c-h> :tabprevious<CR> nmap <c-j> :tabmove -1<CR> nmap <c-k> :tabmove +1<CR> " search and replace nmap <leader>c :let @/=""<CR> " trailing spaces nmap <silent> <leader>dt mq:%s/\s\+$//e<Bar>:let @/=""<CR>`q:delm q<CR> " update, write nmap <leader>u <leader>dt:update<CR> nmap <leader>w <leader>dt:write<CR> cmap w!! w !sudo tee > /dev/null % " window manipulation nmap <c-w>== :set ead=ver ea noea<CR> nmap <c-w>=- :set ead=hor ea noea<CR> " shell nmap <space> :sh<CR> " Plugins " ========================================================= call plug#begin('~/.vim/plugged') Plug 'fatih/vim-go' Plug 'mattn/emmet-vim' Plug 'itchyny/lightline.vim' Plug 'sheerun/vim-polyglot' Plug 'scrooloose/syntastic' call plug#end() " Lightline.vim let g:lightline = { \ 'colorscheme': 'jellybeans', \ 'active': { \ 'left': [ [ 'mode', 'paste' ], \ [ 'fugitive', 'filename', 'readonly', 'modified' ] ] }, \} " Syntastic " let g:syntastic_sass_checkers = ["sassc"] " Netrw nmap <leader>tr <Plug>NetrwRefresh map <leader>tf :Ntree<CR> map <silent> <leader>f <S-CR> let g:netrw_liststyle=3 let g:netrw_banner=0 let g:netrw_mousemaps=0 let g:netrw_chgwin=1 " open files in left window by default " let g:netrw_keepdir=0 " let g:netrw_silent= 1 " fire up the sidebar nmap <silent> <C-@> :rightbelow 25vs<CR>:e .<CR> " vim-go " disable fmt on save let g:go_fmt_autosave = 0 " format with goimports instead of gofmt let g:go_fmt_command = "goimports" " additional highlight let g:go_highlight_functions = 1 let g:go_highlight_methods = 1 let g:go_highlight_structs = 1 let g:go_highlight_operators = 1 let g:go_highlight_build_constraints = 1 " YouCompleteMe let g:ycm_confirm_extra_conf=0 let g:ycm_autoclose_preview_window_after_completion=1 let g:ycm_min_num_of_chars_for_completion=1 " Emmet-vim let g:user_emmet_leader_key='<c-m>' " Formats the statusline " set statusline=%f " file name " set statusline+=[%{strlen(&fenc)?&fenc:'none'}, "file encoding " set statusline+=%{&ff}] "file format " set statusline+=%y "filetype " set statusline+=%h "help file flag " set statusline+=%m "modified flag " set statusline+=%r "read only flag " " set statusline+=\ %= " align left " set statusline+=Line:%l/%L[%p%%] " line X of Y [percent of file] " set statusline+=\ Col:%c " current column " set statusline+=\ Buf:%n " Buffer number " set statusline+=\ [%b][0x%B]\ " ASCII and byte code under cursor " Miscellaneous " ========================================================= nmap <leader>t :call <SID>SynStack()<CR> function! <SID>SynStack() if !exists("*synstack") return endif echo map(synstack(line('.'), col('.')), 'synIDattr(v:val, "name")') endfunc :set guioptions-=m "remove menu bar :set guioptions-=T "remove toolbar :set guioptions-=r "remove right-hand scroll bar :set guioptions-=L "remove left-hand scroll bar 

It's quite big, but the only one mapping related with C-U: imap <C-U> <C-G>u<C-U>. What's going on?