f.lux keeps popping up warning about "Insecure update error!"

[Complexity]

Since a few days now, every few days an untitled window pops up on my screen warning about an "Insecure update error!". The text reads "For security reasons, you need to code sign your application or sign your updates with a DSA key. See https://sparkle-project.org/documentation/ for more information."

With some effort, I was able to trace the owner of this window to f.lux. In fact, it can be triggered by "About f.lux/Check for updates".

I am confused why this warning shows up now, since the last update to f.lux was quite some time ago.

BTW, I am on 10.11.6

Thanks!