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If you have forgotten your username and/or password and you have generated an emergency account recovery key, then you can use it to recover your account.
If you have no recovery key, please carefully read our account recovery policy
Having trouble with the CAPTCHA?
Are you always seeing: "The confirmation code you entered was not valid"?
When this problem has been reported in the past, the cause has almost always been traced to a browser with a malfunctioning extension. Project Euler has very strict security headers and will block any browsers that don't play by the rules. First of all, try a different browser. This should fix the problem.
If you're happy to use the other browser, then the problem is fixed. If you want to use the browser that is not working, you're going to have to find the plug-in (or possibly a security setting) that is interfering with sessions.
We are not able to diagnose your particular problem as everyone's setup is different, but it is usually traced to a broken extension, the browser is in developer mode, or an anti-virus suite that doesn't work properly in the browser.