The launchD is run under the same user, and the loggend in user is also shown as running Plex in activity monitor. So I guess there is no user conflict going on. 
I will however try to disable the launchD and launch Plex Home Theater normally by double click to see if this improves my situation.

Among your tips to improve reception, only hardware failure or battery strength seems to apply in my case. My light situation is not critical at all. I will now replace the Apple Remote's battery as a first step and see if this helps.

I will keep you updated! 
thanks, Oliver.