Anyone has any information about how "fmsgetpasskey" is supposed to work?

I had issues on an FMS19 installation, and after stopping apache I wasn't able to start it without a reboot. The server has an encrypted SSL key, and when starting apache using "HTTPServer/bin/httpdctl start" the log said that fmsgetpasskey returned a bad password.

fmsgetpasskey is referenced in HTTPServer/conf/extra/httpd-ssl.conf:
SSLPassPhraseDialog exec:/usr/bin/fmsgetpasskey

Everything I tried to get fmsgetpasskey to return a password on different installations (sometimes found in /usr/bin/, sometimes in /usr/local/bin/, but always linked to /opt/FileMaker/FileMaker Server/Database Server/bin/fmsgetpasskey or /Library/FileMaker Server/..., a 7MB binary), calling it according to mod_ssl - Apache HTTP Server Version 2.4, I got no output at all.

I guess that some environment gets set when starting it on a reboot, but I found no hint what this might be.