Magdja: Hello everyone sorry in advance for my bad english, I am french. I'm making this topic to help other people who would like to play to Diablo 1 on battle.net once you fixed your port forwarding issue... If like me you got the graphical issue on the login screen for playing on battle.net where all the text box's are empty, you could do as follow... It worked for me and I hope it'll work for you. Keep in mind that my explanation are for Windows 11, I know that you can do the samething in earlyer versions of windows but the method to do it could differ.

Just to explain what we gonna do, the issue for me was that the security of my computer seemed to block the graphical content of the game... Not sure why. It's up to you if you wanna do this since the fix that I found was to drop the security awarness of my computer, which can probably be risky, but it allow to the game to work properly.

1. Go in your windows search bar in the taskbar and tape Security And Maintenance and open it.
2. In the Security And Maintenance window, go in Change User Account Control settings.
3. Drop the little square to "Never notify".
4. Click on "OK".
5. Restart the pc.
6. Open up your game and have fun.

I hope it works for you! :)

PS: If anyone else have a fix that don't require to drop the security awarness, like I did, don't esitate to let me know it would be appreciated, thank you!