Cannot switch to Plex TV Series scanner

Plex Media Server
,
1

Server Version#: 1.25.2.5319
Player Version#: 4.66.1

Trying to switch TV Series scanner to “Plex TV Series” in Manage Libraries → Edit → Advanced

When hit Save - the following message appears:
Your changes could not be saved.

This is Synology NAS install (DS720+/v7.0.1-42218)

2

I think I figured it out by playing with different combinations for Scanner/Agent (TV Series) and it mysteriously offered me to upgrade the libraries and Voila!