phpcd.vim: Channel error using vim 8

The first completion after opening a new file always seems to work. Repeating the same completion, or any other completion, then fails with this message:

Error detected while processing function phpcd#CompletePHP[54]..phpcd#GetCurrentNameSpace[1]..rpc#request:
line    6:
E906: not an open channel

Running ps aux | grep php shows that there are two PHPCD processes running:

wgilk           42770   0.0  0.1  2489520  11080   ??  Ss    8:34PM   0:00.02 /usr/local/bin/php /Users/wgilk/.vim/plugged/phpcd.vim/php/main.php /Volumes/Webserver/league/oauth2-google PHPID json
wgilk           42769   0.0  0.1  2479280  11188   ??  Ss    8:34PM   0:00.02 /usr/local/bin/php /Users/wgilk/.vim/plugged/phpcd.vim/php/main.php /Volumes/Webserver/league/oauth2-google PHPCD json

Sometimes the complete will start working again after some period of time, and sometimes it does not.