I found the answer was to restore the setting back to the defaults but I wasn’t sure where to do that. I found it if you go to Settings, then look under the Plex section on the left side of the screen and select Debug. In there you should see a red button labelled “Restore Defaults”. I clicked that and followed through the other screens to restore things and it all works now. Hope that helps.
Thanks to HALHUB for the answer from his/her post at Plex App for Mac: Custom Filter Error/Library view stuck in error state
