In theory a crash should not corrupt files. But there's the saying about the difference between theory and practice: it's much larger in practice than in theory.
So if the game worked previously but then crashes multiple times in a row, before trying anything else run an integrity check with Steam: right-click on the game (in the list), Properties, "Installed Files" (on the left), "Verify integrity of game files" on the right. This is much faster than redownloading the game and still ensures the game files contain what they're supposed to contain.

Season 26 (2025 March)

Looks like Daemon Tools still/again can trigger the anticheat. Besides closing its user interface, also make sure its background services are not running:

• DiscSoftBusServciePro.exe
  
• DTAgent.exe
  
• DTShellHlp.exe

• WiseTray.exe - the background process of Wise Care 365
• There's a single report about ESET antivirus triggering the anticheat
  
• Some people say SyncError suggested them to to delete initial.pak and shared.pak, then run an integrity check. Given the size of shared.pak, practically this is a complete reinstall with extra steps. Also, an integrity check with Steam should spot broken files without deleting them manually. Trying to find out more
  
• Crazy development: anticheat checks integrity of practically all game files at start. While it supposedly happens in the background, it can cause delays in loading (into the menu, into maps), and it's a constant load until it completes. HDD users are particularly affected, as check can easily last for 30-40 minutes for them, during which time it's a source for random jitter, FPS drops. And that's the good case, when player managed to load into the map without kicked back into the menu.

Season 27 (2025 June)

The strange check has been reworked and thus now the game should run properly from HDD too, though it's not fully confirmed yet.

Other anti cheats: 3rd-party anti cheat clients can trigger the one in this game. It may be a good idea to exit Faceit or Vanguard (Riot) for example.

Streaming/recording: some of them are more intrusive than expected, and will trigger the AC

• Medal - no known fix
• Bandicam - no known fix

Monitoring

• MSI Afterburner actually works, but "Detection level" in RivaTuner has to be set to low
• Process Explorer

Crash for sure

• Cheat Engine, WeMod
• AutoHotkey
• JoyToKey, reWASD

While some may feel it being more recent, this game has an anti-cheat (abbreviated as AC for the rest of this guide) since the 2021 summer update. If Quake Champions crashes during the first minute after you start it, that may be done by the AC. There are no messages, pop-ups, etc, it really just crashes (one may get the error reporting dialog of Windows).
Exact details of how the AC operates are not shared for obvious reasons, what we do know are the following:

• It works in user-space, so it's not a kernel-level thing digging into your privacy
  
• Windows can't be in test mode, use of unsigned / test-signed drivers is not allowed (like InterAccel or Interception)
  
• AC may specifically look for Python running, as there was a large influx of OpenCV-based cheats at a time
  
• Software-originated input events trigger the anti-cheat when they happen. So, AC probably doesn't care if macro software (like AutoHotkey) is just present in the memory, but it crashes the game when a macro gets triggered. Similarly, controller-to-keyboard/mouse mapper software (like JoyToKey) are not expected to work, there were cases of mouse cord tangled into the controller ended up pulling a thumbstick and then crashing the game
  
• Processes attaching themselves to quakechampions.exe are triggering the AC. These are the most common cases, when trying to figure out why Quake Champions crashes, stop all programs offering "integrations" and overlays. This also applies to power tools, like debuggers and advanced task managers (Process Explorer is known to trigger the AC). Some overzealous virus scanners also made the game crash in the past, but that's supposed to be sorted out by now (experiment with skipping the anti-virus as a very last attempt only, even in that case don't disable it completely, but look for a way to make quakechampions.exe an exception).

After getting the game start up properly, one can experiment with restarting the various background applications they can't live without, and find out which is the one that triggers the AC.
For completeness: there are some contemporary (to the launch of AC) tips on Church of Quake[churchofquake.com]. The compatibility checker they offer checks for some of the problem makers, and while the Interception driver is supposed to be the distant past by now, there's an uninstall guide on the linked page (I don't know details, but RawAccel is supposed to be a suitable replacement that doesn't trigger various anti-cheats, not just in this game).

If stuck, one should consider joining https://discord.gg/quake, and contact SyncError in the #bug-reports[discord.com] channel. He will then ask for a crash log, and tell what's happening.

Daemon Tools crashed the 2024 Autumn update, its services in particular:

• DiscSoftBusServciePro.exe
  
• DTAgent.exe
  
• DTShellHlp.exe

Season 25 (2024 December) was incompatible with Windows 10 LTSC 1809 at launch (Windows 10 LTSC 21H2 and Windows 11 LTSC were okay). It got fixed in 2025 January.

Older stuff:

• X-split Broadcaster
  
• Elgato "Sound Capture"
  
• Opening the "Save As..." dialog in various applications, including MSPaint
  
• First release of AC was triggered by some common anti-virus solutions, Windows Defender, BitDefender, Kaspersky.

CPU instruction set
While the developers try to make sure that this game runs on all currently "alive" Windows variants, including their minimum hardware requirements, there were changes, hiccups:

• SSE4.1 support was dropped in the 2022 Summer update, SSE4.2 was required (presumably for its crc/comparison[en.wikipedia.org] instructions). Then it may have got reverted, it's hard to tell now
  
• 2024 Summer update broke compatibility with a number of processor generations: Lynnfield, Bloomfield, and Gulftown on Intel side (most "3-digit" Core CPU-s, and 34xx Xeons), and their AMD counterparts (I won't pretend I understand anything about AMD names, there was stuff like 5800X did not work, but 5600X and 5800X3D did). This was fixed in a mid-season patch

Linux
QC generally works with Proton. Although it's not supported officially, smaller incompatibilities tend to get addressed. These should be reported on discord as no official person is reading anything here on Steam.
No crash, and just a coincidence (applies to many games): recently Proton may start lagging after half an hour or so. Add an empty LD_PRELOAD= parameter as described here[forums.linuxmint.com]

Past problems unrelated to anticheat
Dark Matter Gun (new RL skin from Season 25 / 2024 December) caused an endless loading loop for the 2nd match. This was fixed in 2025 January, together with the Windows 10 LTSC 1809 incompatibility.