Wow, they are really slow at pushing an update. The changes that needed to be made can be found [here]. As you can see the changes have been merged but there is no current build there so I just manually tweaked the current add-on. What follows is what I did that worked for me using a Windows PC.
Grab a copy of the cached add-on zip from Kodi at…
~/.kodi/addons/packages/script.plex-0.3.4.zip
and copy it to your PC and then unzip it. If you have SAMBA server installed in OSMC then you can just open Explorer and type in…
\\192.168.x.x\osmc\.kodi\addons\packages\script.plex-0.3.4.zip
(replacting the ip address) and then just drag and drop the script.plex folder over to somewhere on your PC. Next install notepad++ if your don’t already have it. Open Notepad++ and File>open folder as workspace and select the script.plex folder you pulled from the zip file. Double click addon.xml and modify the version number on the fourth line to 0.3.4.1 and then save and close that file. In the workspace pane right click on the script.plex folder and select find in files. In the dialog box that comes up tell it to find “.isalive()” and replace “.is_alive()” and click “replace in files”. Close the workspace and quite Notepad++. In Explorer right click on the script.plex folder and select “compress to zip” and name it “script.plex-0.3.4.1” and then copy this zip file somewhere that you can access from within Kodi (like the osmc home folder). Finally you just need to go to the add-ons section of Kodi, tell it to “install from zip file” and point it to where you saved that zip.
One final note, don’t bother trying to directly edit ~/.kodi/addons/script.plex as that method doesn’t work.