src-to-pot.sh fails to extract POT with po4a v0.68 or newer since the first source file, nav.adoc, with the message:

You must provide the same amount of master files and localized files to synchronize them, as po4a-gettextize is intended to synchronize master files and previously existing translations. If just want to extract POT files of your master files, please use po4a-updatepo. Please note that the most convenient way of using po4a is to write a po4a.conf file and use the integrated po4a(1) program.

As of po4a 0.68, po4a-gettextize expects a localized doc to be provided along with the master doc (e.g. nav.adoc) to update a PO file. Its documentation (man page) also now states:

You must provide both a master file (e.g., the source in English) and an existing translated file (e.g. a previous translation attempt without po4a).

See offending commit: https://github.com/mquinson/po4a/commit/f2bddaddb79bbcd9bcd6cafa5b1c6a9e68494280