After upgrading YouCompleteMe recently, it stopped working. My first thought was that the libraries needed rebuilding:
python install.py --gocode-completer --clang-completer
The rebuild finished successfully, but each time I started vim I saw a message that the YCM server had crashed. I tried enabling logging with the following commands, but that didn’t work (it was crashing before anything could be logged)
let g:ycm_server_log_level = 'debug'
let g:ycm_server_keep_logfiles = 1
let g:ycm_server_use_vim_stdout = 0
It turns out that the issue was my use of python rather than python2 when building YouCompleteMe. To fix the issue, rebuild the library using python2 rather than python
python2 install.py --gocode-completer --clang-completer
Once this is done, I added let g:ycm_path_to_python_interpreter = '/usr/bin/python2' to my .vimrc everything seems to be working great again.
If you get an error about libtinfo, you may need to install ncurses5-compat-libs via the AUR