For some reason when I run custom cron groups I get the following error (note this is using IIS on Windows which is not officially supported).
php bin/magento cron:run --group=custom
Windows cannot find 'C:\Program'. Make sure you've typed the name correctly
start /B "magento background task" C:\Program Files (x86)\PHP\v7.0\php.exe "c:\dev\bin/magento" cron:run
--group=custom --bootstrap=standaloneProcessStarted=1 2>&1
If I run the cron tasks which are part of this group as the default group I do not get this error.