Minecraft LAN World Windows and Mac?

My brother (mac) and I (windows) want to play together on a LAN world. I have released it as a LAN world and in my opinion the firewall setting is correct. Doesn't it work because he is playing on Mac and I on a Windows computer or why else?

Important points:

Same Minecraft version
You MUST be on the same network
NO mods may be installed

But it's not a problem that he has a Mac, is it?

This should normally not be due to the OS, so no

Mh man even issued my firewall

Mods may already be installed, however the other person MUST have installed the mods.

Try to turn off the firewall on the Mac PC too

So we both have no mods anyway, he said he saw them briefly but didn't think of it because of a client error or something… Both started again and now he can't find it again