Hi nikinp, I originally followed this guide and was having issues with transcoding via ffmpeg. If your issue is the same as mine, then I can say that the issue has been fixed in the latest version (
230923-230923)! 
The issue was related to the -r option being used in conjunction with the -vsync option -- that's a no-no, at least with the version of ffmpeg I was using.  I am not a freebsd  or ffmpeg expert, but I'm sure that whatever ffmpeg is currently shipping with the 12.4 jail is what I used.  Here's some more info on that issue and the resolution: 
https://github.com/photoprism/photoprism/issues/3659#event-10455019855..
In any case, all of my issues were resolved by installing to a 13.2 jail, setting my repository to "daily" rather than quarterly (in order to get the very latest gettext and ffmpeg), and compiling using the makefile at 
https://github.com/huo-ju/photoprism-freebsd-port