Try running pkg install libidn2 in the shell for that jail. After a jail restart plex should pick back up. It seems like there were come components missing with this build.
note - if that doens’t do it also run pkg install p5-Locale-libintl — some people need it, some don’t it seems. I needed both.
see this thread for details - https://forums.plex.tv/t/1-19-1-2589-not-working-on-freenas-after-upgrade/565650/22
