Minecraft local server: World doesn't load

I played on a private Minecraft Server downloaded directly from minecraft.net with my friend on local LAN. This worked perfectly. But when I try to join the server over the internet, I see the server in my Server List and I can join, but the world isn't loading at all (seems like I'm falling) and I get kicked after about 10 seconds. If i write something, the server also receives it so it seems like the server isn't able to answer the clients properly.

I set up a port forwarding for my PC on Port 25565 and set up my firewall rules (I also tried turning the firewall off for testing but this didn't change anything).

I don't want to use things like LogMeInHamachi or ngrok and it would work but I think it should be able to work without these tools as well but I cant find out why this happens. I tried joining to my server on my PC with a laptop from a different Internet Provider but it didn't work.

I assigned 4GB ram (I have 16GB) and activated UPNP and tried using a DNS but all this things didn't help. Does anybody know what it could be that others can join but cant receive any further data from my Server?