Thank you for the screenshots and the explanation of the problem. It helped me a lot. I made a n00b mistake. Forgot that lists work with negative indexing too, so this way the board was basically a sphere. Once it stopped checking if the field "exists" and concentrate on the range of possible fields it works... i hope.

Sorry. I have no idea what you're talking about. I don't play Reversi that much, I like the programing challenge it gives me.
For now it only finds current players disks and from them sees in which are possible moves and which disks will be taken over in the process. I would be grateful if you could show me in the direction where these algorithms can be found, or if you would be kind to explain it here on tmo, so others can learn too.

Fixed it. Forgot to reinitialize the players too.

Sure, I would like that. I like to learn about new things. If you would like and know python you can create the AI script. You can see the random_computer class in the source code or send me a pseudo code and I'll implement it. Also for the help if you would like I would add you to the developers list.