Great! Thanks so much for sending me the exact error image; a picture makes all the difference in finding the root cause of the problem.
The fact that you were able to choose the 2560x1440 resolution is a good sign, as it means the game was able to identify it and list it as an available option. However, the failure occurred when the game tried to revert to your original resolution, as you correctly pointed out.
After looking into the situation, I believe I've solved the resolution recovery issue, especially for ultrawide monitors like yours! :)
This leads me to an important question: on the startup screen where you choose the resolution, does your default 3440x1440 resolution appear in the list of options?
Regardless of your answer, I've implemented a new security layer that now allows the game to identify and recover your default resolution, even if it wasn't initially listed.
I wanted to release this fix immediately, but it would disrupt my current workflow. So, I'll include this improvement in the next update. As always, I'll write a detailed changelog and let you know here so you can test it and give me your final verdict! Like I said, I believe I've solved it, but I'll only know for sure with your confirmation. :D
And, while I'm at it, one more question: how does the game look when running in 800x600 with the SCREEN option checked? Did it look good, filling the entire screen?
My idea is to replace the "800x600" option with the text "Fullscreen," as I believe most players prefer to run the game at the highest possible resolution and don't realize the fullscreen dynamic that the 800x600 setting offers.
Again, thank you so much for your help!