The error in the log is:

System.IO.FileNotFoundException: Windows mono path not found: C:\Program Files (x86)\Steam\steamapps\mono\mono\mini\mono.exe

  at Mono.CSharp.CSharpCodeCompiler..cctor () [0x00000] in <filename unknown>:0 

This comes up a lot these days. It pretty much means Unity is unhappy, but that could be for a lot of reasons.

You can read one thread about it here. In that case he fixed it by changing the galaxy density.

Updating to the latest version of Windows 10 has fixed it.

You can also try set "-force-opengl" in the Steam launch options. To do so, right click the game in Steam, go to properies, then click on "Set Launch Options...". That has fixed it.

If you are missing Mono you can also try installing it then copying the Mono folder to steamapps. You can find the installer for Mono here. I haven't actually seen this fix or the ones below work. The real problem is not that you are missing Mono, it looks for something in Microsoft .NET and when it can't find that it resorts to looking for Mono.

This could be caused by not having Microsoft .NET Framework 4.5.2. You can download it here.

You can try installing C++ Redistributables (vcredist). It can be found here: x86 (32 bit OS) x64 (64 bit OS) 

You can also try going to the "C:/Program Files (x86)/Steam/steamapps/common/Endless Space 2/EndlessSpace2_Data/" Folder and deleting the mono folder then verify the games files with steam. It should redownload the folder correctly.

The devs have commented there is a known issue with Unity and non-English characters, they said: Some people using non-english OS and international characters in their steam path could not launch the game. Such characters are badly handled by the low level layer of Unity, Mono. We have performed a workaround in the code, (better than having to reinstall Steam to a path without accents)  waiting for a cleaner solution from Unity.
I have seen others propose a fix to this that involves deleting fonts which are corrupted. Are you using a non-English computer?

Windows 7 x64, running in English.

I've got Microsoft .NET Framework 4.7.2 (blocks install of 4.5.2), and Microsoft Visual C++  2010 x64 Redistributable 10.0.40219 (and the x86 version) already.

This was a pretty late-game issue, and the first time I'd seen it, went back and finished it with no further issues.

Dunno if this is related, but something I had noticed is that sound effects for the weapons slowly turned to static before cutting off completely. Like it's not doing garbage collection on sound effects once they've played, so the more shots fired the worse it gets. Seems to happen most in Overview mode, I've seen this happen when GUD was streaming the game as well.

Windows Firewall and Microsoft Security Essentials. Clean installed the GPU drivers back in January with a DDU wipe.

DxDiag.txt

This is the first time I've gotten the crash. The audio static has been a persistent issue for months.

Here's the GUD video with some static, unfortunately he cut off the sound pretty quickly:
https://youtu.be/-lfLC0g9QVo?t=1034

Another crash today, running the battle on 4x speed this time.

Version: 1.5.37 S5 Gold-Public

GPU drivers are the most recent AMD November 2020 drivers: "radeon-software-adrenalin-2020-20.11.2-win7-64bit-nov18".

> Unity Player [version: Unity 5.5.1p4_f10174ef3e2d]
>
> AkSoundEngine.dll caused an Access Violation (0xc0000005)
in module AkSoundEngine.dll at 0033:cae5c2e0.
>
> Error occurred at 2020-11-30_223250.

2020-11-30_185356.7z

DxDiag.txt

Another crash, running 4x speed again.

Version: 1.5.48 S5 Gold-Public

Same GPU drivers, they stopped updating my drivers for Win7.

> Unity Player [version: Unity 5.5.1p4_f10174ef3e2d]
>
> AkSoundEngine.dll caused an Access Violation (0xc0000005)
in module AkSoundEngine.dll at 0033:b824c2e0.
>
> Error occurred at 2022-01-23_063250.

2022-01-23_060056.7z