I installed my plex server behind an apache ssl proxy. This works great with the web client, it also works with the IOS client and Plex Home Theater.
It doesn't work with RasPlex though. I guess it is because ssl support is lacking from the openelec version used. A wget started on my Raspberry to a https side also complained about it not being a http / ftp link.
Since Plex is going to SSL one day (hopefully soon) it would be good if RasPlex would also be SSL enabled.
Request remains, but since other clients don't seem to work either and RasPlex follows PHT I understand a little waiting is needed.