5 sec flare near Arcturus
My first thought is that you saw a flare from a satellite, meaning a favorable reflection off of a solar panel or something. I use the software "Heavensat", and it shows the satellite Nextsat (Norad number 30774) passed very close to Arcturus at 23:40 pm from your location (45.7288° N, 24.3784° E). Note that I am assuming that your time is 3 hours ahead of UT (Universal Time).



I do not know if Nextsat typically flares, although reflections from satellites are common. Nextsat is in a low altitude orbit, so its motion would normally be detectible. Perhaps the duration of the brightening was shorter than you estimate (so that the motion was not detectible), or it was so unexpected that you did not notice the motion.

My second thought would be a head on meteor like you suggested.