PlexkodiConnect

You have to ask the addon author for support. There are some forum threads as well:

Plex Kodi Connect - Der inofizielle Nachfolger von PleXBMC - Addons & Repositories - Kodinerds.net - Deutschsprachiges Forum zum Kodi Entertainment Center

PlexKodiConnect: supercharge your Plex - Kodi connection

PlexKodiConnect: let Kodi talk to your Plex — Plex Forums

'Croneter' the user on the plex.tv forum is the maintainer and he has his own repository at github:

GitHub - croneter/PlexKodiConnect: Plex add-on for Kodi

Depending on where you get this addon from (as it's not in the official repositories...neiter at kodi repos nor at LE), you have to contact either them or the maintainer for support.