Just to save everyone some trouble, PLEX version 1.15.6.1079 does not resolve the issue with stopping/uninstalling the software on ReadyNAS.
Model: ReadyNAS 214
Firmware: 6.10.0 Hotfix 2
Revert to 1.14.1.5488 and everything works normally. Still no statement from the dev team that I am aware of regarding this issue.