Thanks for all your replies!
It worked now, most of the time ...
... yes it might be a bug that occurs occasionally. When it doesn't work, the Pylon gets initialized (5min timer appears), but never activated, so it remains in that stage.
But now I see it working most of the time (5 out of 6 attempts), i.e. I could activate the Pylon and everything worked as expected:
The one time it didn't work, I could wait until the 5min timer was over and then do it again and the Pylon could be activated then.
daytime was no factor in the past
Yeah, likely it still isn't. Now I tried it during sunset/daytime and though it worked most of the time, it also didn't one time. During nighttime it didn't work for me (did 5 attempts via relog or waiting until the timer was over), though this might be just coincidence.
5 minutes is too long. I can't remember the exact times, but something like 2 minutes each. Each time you charge one, the total time goes up by 2 minutes[...]
If you saw 5 mins on the timer, you must have charged at least 3 pylons!
5 minutes is the initial time when shooting the first Pylon, before it gets activated. After activation, the 2 minutes are added, so the first Pylon gives 7min in total if everything works.