flickering/flashes every other second

Hi, I used to have a configuration from like 2019 (with a 720p grabber). However, I wantes to upgrade to 4k of course.

So, I read an article, which suggested a Rullz 4k 60HDMI capture with loop. This is working.

However, when I want to set my new configuration, sometimes the LEDs flashes, which is very annoying of course.

I created a little video: https://imgur.com/a/FK0Glbo, which shows the annoying flashing.

I've properly grounded the Raspberry pi 3 twice with the ground LED input cable.

My own thought is the signal cable, maybe a condensator anywhere? But when moving configurations, nothing happened to the signal cable...

Thanks!

Alright, sorry.

My setup is:

*197 LEDs behind my TV;

*Raspberry pi 3

*Rullz 4k 60HDMI capture with loop

*signal with a resistor (old one, don't know the value anymore), going directly in the Pi3 - GPIO18

* + going to pi3, capture, LEDs

* ground going from 5v to pi, capture, LEDs

* ground going from LEDs to Pi3

Actually, looks similar like this picture below (credits: https://peyanski.com/tv-ambient-lig…i-and-hyperion/)