Remmina: "Protocol plugin RDP is not installed", even after purge and install

I just ran into this myself (Kubuntu 14.04 LTS). I had the basic remmina package installed, but I wanted to add in all the optional protocols. I think I figured out why rebooting fixed the problem for you: the remmina process persists. It seems that when you close Remmina, you just close the GUI - the binary is still running, and it needs to be stopped and restarted to be aware that you've given it new plugins. At least, that's what happened in my case, and it explains what you observed - a quick killall remmina at the beginning saves a reboot cycle.

So, here's my method for a fresh Remmina install, with all the client protocols, on any *buntu variant, without nuking any saved connection profiles!

1. Clean your environment:

   sudo killall remmina
   sudo apt-get purge remmina* -y
   rm -Rf /home/`whoami`/.remmina/remmina.pref
   

2. (Optional) Add the Remmina repo to get the latest build:

   sudo add-apt-repository ppa:remmina-ppa-team/remmina-next
   sudo apt-get update
   

3. Install all the things:

   sudo apt-get install remmina* -y
   

Well, perhaps a reboot would've done the trick.

I removed Remmina from my machine yesterday and I shut my machine down at the end of the day. After starting it back up this morning, I installed Remmina again.

sudo apt-get install remmina remmina-plugin-vnc remmina-plugin-rdp

After this, I was able to connect to Windows machines again; no complaints about missing plug-ins.

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While this has worked for me for now, I won't accept my own answer, since it just happened to work; I have no clue why.