Which path is used for authpipe program

I moved from a working courier installation with Ubuntu 14.04 to Ubuntu 16.04.
My configuration use authpipe for authentification with a self written authProg.

Ubuntu 14.04 use a non default path for the authProg: /etc/courier/authProg

In Ubuntu 16.04 I have installed courier and adapted the configuration (use only authpipe), so its matches the old installation. But now I can not log into my accounts.

After inspecting the logs it looks like the authProg is not found. When the authdaemond is restarted following message is written in the system log:

Starting Courier Authentification Daemon...
Started Courier Authentification Daemon.
authdaemond: modules="authpipe", daemons=5
authdaemond: Installing libauthpipe
authdaemond: file not found

So I thought the path used for the authProg have changed, maybe to the default one of courier: /usr/local/etc/authlib/authProg
I have copied the authProg also to this location, but nothing changed.

Finally the question is still open, which path is used in Ubuntu 16.04 to run the authProg?