Unable to cast to Rasplex clients

Rasplex was based on “Plex Home Theater”. This has been deprecated and left unsupported for years now.
I think development of the rest of Plex has now reached a point where its no longer compatible with such an old client codebase.

IMHO, your best course of action is switching to LibreElec and the Plex4Kodi plugin by @panni : PlexMod for Kodi (18, 19, 20, 21)