Liberation sans unavailable as substitution font

Dear GanttProject Support,
I’m working on macOS Mojave (10.14.6) and using GanttProject 3.1.3102 (recently downloaded). During the last year, I’ve been able to export in PDF format using font Liberation Sans. Today, GanttProject tells me that Liberation Sans is not available anymore as substitution font (it’s still installed on my computer) and the exported PDFs show no text. I’ve tried using Helvetica and Times New Roman (changed in both Settings and Substitution font), but it didn’t work.
Do you know which font should I use?
Thank you very much in advance.
Kind regards,