Weird Screen Display - Start screen and settings unusable, making the game unplayable

[Elebato]

Since some versions of ET Legacy, the game is unusable for me. Whenever I move the mouse, every single move stays permanent on the screen.
The same happens when opening different settings. They overlap, that means when leaving a setting, it still stays on the screen and will be overlapped by the next one.
This also happens to the screen where I try to choose a server for playing.
The ONLY time everything looks fine again is when a dialogue is shown like the dialogue for exiting the game (Screenshot 5).
Screenshot 1 was made after starting the game before moving the mouse, Screenshots 2 through 4 show the described behavior.
I deleted all game files including the folder at ~/Library/Application Support/etlegacy and installed the game again several times.
Version 2.7x didn't have this problem, I think 2.75 or 2.76 was the last one working correctly, but I am not sure.

My system:

• Mac Pro 4,1 (Early 2009, Single CPU Xeon X5690 3,46GHz) with 32GB RAM
• OS: macOS 10.14.6
• GPU: Sapphire Radeon RX 580 Pulse 8GB
• ET Legacy v2.80.2

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