Rasplex hasn’t seen an update in years, by now.
It is to be expected that the server software will evolve to a point when it will eventually no longer be usable with ancient clients.
While there are plenty of other compatibility features and still-maintained old APIs in Plex server at this time, there won’t be made any additional accomodations in the server for such old (and unsupported) clients.
The cure is relatively simple though: Get a linux distribution for the RaspberryPi which is specialized in media playback. The best known is probably https://wiki.libreelec.tv/
Then install the PM4K add-in, which provides the compatibility layer Kodi<->Plex
PM4K / PlexMod for Kodi (18, 19, 20, 21)
The result will be very close, but better, to how Rasplex looked and worked.
