Others (including myself) have had similar issues.
Was Plex running with 18.2.2029 before you upgraded? Have you recently upgraded Plex? …some have found that the problem was that Plex was still running older instances of Plex at the same time as 18.2.2029 and they had to “kill -9” them.
Others realised (too late) that QT 4.4.1.117 was still in beta for their particular QNAP (mine is OK - I have a TS-453Bmini).
J.
[edit:’ Forgot to add that I do now have 4.4.1.117 running with Plex 18.2.2029. I think my problem was that I upgraded QTS and then lex and had the multiple instances running. I solved my problem by downgrading QTS and then re-ugrading QTS which (I think) effectively killed the older instances of Plex running when the QNAP restarted.