I've read that (Using millis() for timing. A beginners guide) That was also something I thought about, I forgot to mention.
Unfortunately it doesn't change the weird behavior.
However I found a lot of related topics in this forum that describe the same problem.
Like this:
I actually wanted to avoid the datasheet of the max7219 and try to figure out everything. I was hoping I could just use a ready to use library. But I think it's not uncommon that this chip is not working "out of the box".
I will try to use the SPI way in this answer.
I am very confused. In some threads I read something about the capacitor. I'm not into electronics, so I don't know how to make the circuit yourself. But I am using a ready to use module made by TRU Components.