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The April 22 2020 update to version 2.5.6 broke fullscreen. As I reported back then, the game only starts in windowed mode and ignores the fullscreen setting. Today the update to 2.5.8 was released. Unfortunately it didn't fix this glaring issue.

I hope the fullscreen support gets fixed as an urgent priority. I didn't pay for a game that only works in windowed mode, with Windows task bars and window top bars breaking the immersion. Especially as this issue was caused by an update, and we know which one, it shouldn't take too long to track what part of the update needs to be rolled back and re-implemented later in a better way.
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I have the same problem.

I now disabled automatic updates and was able to get the fullscreen working be downgrading to the older version 2.4.1.

The next newer version I could select is 2.5.5, but that one is broken and the upgrade does not succeed. The other two newever version I could upgrade to are 2.5.6 and 2.5.8 (the newest one) but for both versions the fullscreen mode is not working.