I had the same issue. Work around is to use no display:
$ pyferret -nodisplay
yes? plot i[i=1:10]
yes? frame/file=capture.png
That should work.
Same problem and same workaround here. PDF output has had the same issue for a long time, but PNG output has started to have it for some time, too. Currently I don't have access to pyferret but I think I'm using the latest version. The operating system is macOS Catalina. (Some of my machines have migrated to Big Sur but I haven't used the FRAME command on them.)
Regards,
Ryo