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League of Legends error

Critical error: an overlay or stale cache is blocking League

League clientLaunch

Riot's name for this error

A critical error has occurred

What it means

Riot blames programs with in-game overlays for this one. It stops you getting into League.

What to try first

Fully quit anything with an overlay (Riot names Steam, Origin and Discord), then delete League's Config and data folders and relaunch.

Is it you or Riot? If lots of players are hitting this at once, it's probably the servers. Check League server status before you start reinstalling things.

Two Riot articles, two angles

Riot's League error table was the first to cover this. It pins the error on overlays: software that draws its own layer over your games. Riot names Steam, Origin and Discord as examples. The fix there is to close those, then clear out League's local settings.

A newer Riot article, Troubleshooting Game Launching Errors (updated May 19, 2026) gives system errors and critical errors a shared section with a longer list. It starts with the cache in your AppData folder and ends with a full reinstall of the game and Vanguard. Both are below, the short fix first.

The quick fix from Riot's error table

  1. Quit every program with an overlay. Closing the window isn't always enough, so check the system tray too.
  2. Open the folder League of Legends is installed in (by default C:\Riot Games\League of Legends) and delete the Config folder and the data folder. Riot doesn't spell out the data folder's exact name, so leave alone anything you aren't sure about.
  3. Launch League again.

If that didn't do it

Here's the order from Riot's launch-errors article:

  1. Clear the AppData cache. Open C:\Users\<you>\AppData\Local (typing %localappdata% into Win+R gets you there faster). Delete the game's folder there, then go into the Riot Games folder and delete the game folders inside it too.
  2. Do a clean boot, so only Microsoft's own services start with Windows (in msconfig, tick Hide all Microsoft services before you disable anything). Leave vgc enabled so Vanguard still runs.
  3. Add firewall exceptions for the Riot Client and the game. If you run third-party antivirus, switch it off for a test.
  4. Update your graphics driver. Riot's crash guide points NVIDIA and AMD owners to each maker's own updater, and Intel's too.
  5. Reinstall the game and Vanguard.

The same Riot article sends a "Failed to launch: something unusual happened" message to its own article, which has a different list.

Other wordings of the same error

  • A critical error has occurred League
  • League critical error
  • System Error/Critical Error

Source: Riot Games Player Support, League Error Codes and Solutions, updated May 19, 2025. Riot's steps are paraphrased here, not copied; checked September 25, 2026.