There's a bug currently open in Debian (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1025782) where gpg-wks-server fails at the sendmail-submission stage, with mysterious errors. It appears that since https://dev.gnupg.org/T4886, gpg-wks-server detects sendmail's location at compile time, however Debian have somehow managed to build it with sendmail's location set to the empty string. This scenario should either a) fail at compile time or b) fall back at runtime on the previous behaviour of looking in a standard place (/usr/lib/sendmail).