fwiw: just got a response from BinaryNights. Yes, it's a Forklift3-Sonoma compatibility issue, and no, there won't be another update for 3.5.8.

Same here. And the fun part is: ForkLift 2.6.6 (the old one which they gave away for free on the Mac App Store) seems to be working perfectly, even on an Apple Silicon Mac running macOS 14.3.1 🙃 It's most likely a ForkLift3-Sonoma incompatibility btw: there's no ftp cli client around any longer, I believe they ditched it with Big Sur alongside PHP iirc. But openssh works like a charm. I also dropped them an email, but I don't really expect them to respond.