Neopixels not working with MEGA but working with UNO.

Me and a friend have been working on a board game table for some time now, and we are trying to get the lights to work properly.
Our idea is to control the lights via Bluetooth from an app, and we got it working fine, but we think we ran into a RAM bottleneck of the UNO so we decided to upgrade to a MEGA for the extra memory.
However, it seems that the lights do not work properly with the MEGA, they flash in a crazy manner when we try to get them to do what we want, no matter how simple the code.
What we tried:
-changing wires
-changing the code (making sure we are using the right lights type, pins, number of lights, etc.)
-different power supply
-resistor between data and lights
-making sure everything is grounded

We ran out of ideas so we decided to see if anyone could have any ideas what the problem may be. In the post I am attaching a schematic of the system. (the lights are ws 2812, and the UNO in the schematic is replaced by the MEGA, with either 5v or 12v connected as a power supply, neither fixing the lights)

Perty picture but could you post a real schematic showing all power and ground connections as well as the interconnections. Include links to the hardware parts. Don't forget to post the code.

What is your software. You will need to use one of the different initialisation methods when you change to a Mega.

You don't seem to have any capacitors on the LED strips and there are no series resistors in the data lines. This could also cause the symptoms you are seeing.

The Mega is more critical with its voltage than the Uno. Measure the 5V pin, it should be above 4.5V.